Does anyone here homebrew any kind of additional negative effects to Critical fails, or do you just let it fail?

Personally, I think having an attack miss is punishment enough, but my players all kind of prefer Critical Fail punishments (weird, right?) so I homebrew it that crit fails open the character up to an Attack of Opportunity from the target they're attacking. So their enemy is capable of attacking them, still has to use their reaction to do so, which can result in interesting options for the players now that they know that enemies reactions are spent.

I do also treat 1s rolled in ability checks as automatic fails... because if their modifier is high enough that they would still succeed even if they rolled a 1, I just don't make them roll in the first place. Still, I don't punish my players beyond just failing to do what they were attempting.

I am very mean with natural 1s and very generous with natural20.

In combat, you will hit an ally. Initiative, you will play last, ability check...something bad will happen. I started doing that after learning about another system. The only time I don't do it is when I don't have any idea or when a nat1 already means something else, like for the gunslinger's gun and the misfire rule.

As for what bad thing can happen, I don't want it to be too bad. Like a nat1 on a pickpocketing attempt would either have you fall on your target anime style (but still giving you an occasion of RP your way out of it) or you getting confused and putting money in their pocket instead of taking money out of it.

I have had crossbow strings break or someone take a bad tumble while climbing, when natural 1s have been rolled.  I don't use it all the time, but to me, it adds flavor to the game.

Same for natural 20s.  Every now and then, something "extra" happens, if it really advances the action of the moment or the richness of the story.

Yep! Fumbles are great. I see dnd as 'action comedy' and nat 1s are where the comedy comes in.

I dont use them as "punishment" tho as much as they try force a tactics change. For example, a weapon can become temporarily unavailable (bow string snap, weapon drop, weapon miss and stuck in wall) so they need to switch to a back up weapon. Or their mobility can be compromised (slide to a different square, fall prone, have to go chase the weapon that slipped out of your hands etc..). This can be enjoyable because it forces the party to mix things up. 

My fun way to think about what happens in a fumble is that the character is wildly successful at achieving the opposite of what hey were intending :p

One thing- for characters with multiple attacks my houserule is that you can only fumble if you role a nat 1 on your *last* attack. This is to make it so that experienced fighters with multiple attacks dont fumble more often than newbies.

I tried it and I hated it (as both DM and player).

The Cons, IMO:

1) Yay brand new magical item!  Nat 1… dropped and taken by the enemy :(

2) Number of attacks per round influences your number of nat 1 rolls.  Spellcasters that use save vs spells only, are immune to it.  Fighters at level 5 and 11 will hate their lives.

3) Good or bad, battle does become a comedy.  That can ruin your campaign or enhance it.

your mileage may vary!

My players like combat fumbles, I'm on the fence about them.  I like the 1 as autofail on skill checks, what I usually do rather than have a necessarily "spectacular fail" is the 1 creates a complication to the task or environment (I picked that up from some iteration of the d6 system that the first Star Wars rpg was built around).  For instance, let's say a Ranger is trying to get around the city of Flotsam while under Dragonarmy occupation.  The Ranger disguises themself in the armor of a Dragon Army officer.  Rolling a 1 on a DC 10 disguise effort and you're proficient with a bonus to usually get this automatic, yeah the armor does a good job convincing the occupying army you that your rank gives you free access to the whole city, but turns out one of the Dragonarmy leaders is actually your former lover, and she's definitely going to see through the charade.  So Tanis the player is still in disguise, but there's this new factor complicating the player's scouting around.  Sure complications may derail the intended direction of the game, but I like it when the game goes in a direction no one expected.

Instead of fumbles, I had a DM that made a chart of what he called battlefield events. Things like a wall would crumble, or a sinkhole would open, or a swarm of stinging bugs would get stirred up and create a zone of some sort.

It doesn’t really damage the person who it happens to, but it makes the fight a little more dynamic and forces people to move around a bit instead of just the melee guys slugging it out. It can end up helping or hurting either side since it’s kind of random. 

It's all thematic for me: "nothing but air," "massive whiff," "missed by a foot," or "not even close." Adding consequences just favors casters who rely on attack rolls much less often in combat, and it usually comes across as silly.

I use a table for critical fails in combat...my players roll a d20 after a crit fail, about 45% of the time the attack just auto misses, 50% of the time something additional (and bad) happens, and on a 20, they get to reroll their original attack.  It has different consequences based on whether the attack was melee, ranged, or spell.

I balance that my having crits do max damage + dice, rather than 2x dice.  The d20 gives and takes away

Quote from NaivaraArnuanna >>

It's all thematic for me: "nothing but air," "massive whiff," "missed by a foot," or "not even close." Adding consequences just favors casters who rely on attack rolls much less often in combat, and it usually comes across as silly.

The imbalance between casters and fighters can easily be remedied by treating natural 20's to save against it in the same way as natural 1's.

I intend to treat natural 1's (and natural 20s to opponents saves) more in an environmental way than anything, if possible - you make a greataxe attack, nat 1, miss and hit a tree. Then I'll roll an initiative step for the tree to fall over and a random direction for it to fall in next turn. Could be a statue falling, a pot of oil being shattered - general carnage, really - the opposite of a controlled fight. In a dungeon setting I might have their weapons collide with an incredibly loud clang, alerting other enemies further along the route. I once player where I lost my axe to my first attack with a nat 1, and it kinda sucked, really. I spent my next turn getting it back, which took me out of the combat. I'd only do that sort of thing if it was a silly decision in the first place (EG attacking an iron golem with a regular axe, which wouldn't do anything anyway, might cause a nat1 to have the axe sent spinning from your hands).

Personally, I don't use critical fails for some of the reasons mentioned above.

1) The more attack rolls you get to make, the more likely you are to harm yourself every turn. The level 11 fighter or warlock casting eldritch blast are much more likely to hurt themselves than a level 1 character (or a level 11 wizard casting firebolt - since it is dependent on the number of attack rolls they get to make in a turn and nothing else) - they actually get more dangerous to themselves and others as they get more powerful. Make sense to me? No. So I don't use them.

2) A 1 is an auto miss. Sometimes, I will include some narrative description to make the miss sound impressive or look cool but mechanically there is no need to make misses worse than they already are.

I don't auto fail on skill checks either - a 1+mods either succeeds or not. Another poster mentioned having a 1 on a skill check auto failing but not getting the character to roll if they can't fail the check - which doesn't make any sense to me since in that case a 1 would fail anyway if it was possible for them to fail in the first place. Otherwise they would not have had the character roll.

3) Fumble tables are invariably either too devastating in their effects, too boring, or meaningless. The first time a character chops off a limb with a critical fail folks might take note. When the character is a level 20 fighter, rolls 4 '1's in a single combat round and turns themselves into the knight from Monty Python with no arms and legs .... well, you get the idea. It isn't comedic at that point, just a waste of play time for the folks at the table.

I've tried fumbles, and extreme failures, but I find that the cost of failure alone is enough.  Adventurers are professionals in a fight, hitting their own friends, throwing their weapons across the room, breaking their equipment: these are all just adding insult to injury and can be immersion breaking depending on the tone of the game.

I let it fail, and I describe a spectacular failure, but in a way that maintains the integrity of the character and the game.  There are of course times where failure is comedic, and in those situations, i'll lean into it a little more, but it's when the table is usually leaning into it as well.

My combat scenarios are hard enough that an additional added danger of a 1 causing bad things to happen just adds too much randomness into the mix. I will however work ones into the storytelling narrative of the situation and, if a player has a run of bad luck, I might make something happen for comedic effect but this will be in the moment and based on where the party is in that moment in combat. 

We RP the nat 1, but other than "you miss", there is no mechanical consequence. I might narrate that the character tried to swing and dropped her sword, having to waste her round picking it back up again, but there are no penalties, no Rolemaster-style fumbles like she cut her own toe off when she dropped it, taking 1d6 damage and bleeding at 1 hp /round for 10 rounds or any of that kind of deal.

Improv it! Make the bugbear's javelin ricochet and hit themselves! Have the player leap halfway across a gap, then have them fall straight down cartoon style! Have fun with it!

Nat 1s make great comedic fails, and nat 20s are awesome moments, so get as much fun as you can out of them.

My players now all grin at critical fails and smile with nat 20 rolls.  Nothing over the top, I use % for critical fails, giving them, depending on situation, a 5-15% chance pf hitting an ally if they're in the area, or having a weapon issue (broken bowstring, fumbled sword) or a control problem (overextended, Acrobatics check to avoid falling over {usually at a DC of 13})

Nat 20, I have adopted something I had heard of before, and was re-advised of before beginning my latest campaign, one of the dice rolled becomes a max damage roll.  We play in person so we go with colored dice usually and can call "Red" as we throw the pair, to say which becomes max.  Not many things worse than hitting a crit and rolling a pair of 1's.

I've played in a game that used it and I hated it.

All it does is punish those who played fighters, espeically high-level fighters.

My high-level ranger? Three, four, sometimes five attacks a round, which meant on average one fumble every four to five rounds combat. Since some of the fumble results were "throw your sword 15ft away from you" it basically meant I was getting screwed over almost every combat.

The high level bard and wizard? Zero chance of fumbles. 

Quote from iconarising >>

I balance that my having crits do max damage + dice, rather than 2x dice.  The d20 gives and takes away

 
This benefits Rogues and Smiters in a HUGE way. 

A level 10 rogue Sneak Attack average crit is:

weapon 1d6 + 5, sneak 5d6, crit 6d6 = 47

With perfect crit roll, it’s 1d6 + 5, sneak 5d6, + 36 = 62. That’s a huge difference.

Quote from Lyxen >>Just a point, by chance, I play two halflings (images in a shadow mirror) in the Eberron campaign where our DM is one of our DMs that does this the most, and having fumbles on a natural 1 makes that race absolutely OP (and I also have the lucky feat, which makes it even more overpowered).

 And Barbarians too, that get reckless attack.

I don't do anything mechanically. I think it's pretty ludicrous that a highly experienced fighter would accidentally bash themselves around the head or hit a compatriot a full 5% of the time.

With that said, I do try to have my cake and eat it, because sometimes I do describe the fail in a humorously clumsy manner just for the giggles. Whether the fail is due to external influences or comical clumsiness depends on the feeling around the table.

"You nock your arrow, but just as you loose the sun reflects from the Goblins scimitar and flashes in your eyes. In flinch instinctively and your arrow ends up embedded in the ceiling 10 feet wide of your target."
"You spin your throwing hammer, preparing to loose a mighty attack, but fail to take into account the doorway you are in, and it smashes into the frame, jarring your arm. Your plate mail rings faintly from the impact."
"You raise your morningstar above your head for a mighty overhead strike, the chain and ball hanging behind you. As you begin the downward strike, one of the spikes catches your belt loop, and you succeed only in giving yourself the worst wedgy you've had since you were in school back in Waterdeep." 

We also roleplay massive/crappy damage rolls. I had just role played high damage rolls Ray of Frost, describing a larger than usual ray blasting across the battle arena, the enemy freezing solid and then falling and shattering (a la T2). Next turn the wiz rolled a one on RoF, and one of my players coined the fact that they basically hurled an ice cube at the enemy, and that's now kinda stuck.