People are Human, and for example, forgetting that one is in another room and discussing or using knowledge with other players that is exempt to the player's character can happen by accident.

I've seen a streaming group that would rely on audiences' prior suggestions for "discouragement" and, after a review by the producers, had a RNG randomly pick one if a player acted on metagame knowledge. They put the chosen consequence on a digital monitor among animated smoke with a reflective covering, dressed up to look like a mirror. Nothing dire or extremely debilitating and sometimes something silly and harmless as having to roll using a different hand than normal while sitting on the original hand or as serious as disadvantage on all rolls for 30 real-time minutes (which can end up being a round or two if in combat or a few RP-related checks out of combat).

So...

Brainstorm some ideas to discourage metagaming in a fun way. What gentle but firm and fun reminders against metagaming can you imagine?

Open to players, DM, voyeurs, and everything - from past experiences to new ideas. If you are okay with metagaming, feel free to throw in an idea as if you wanted to discourage it just for fun.

Lock the metagamer in the stockade and have everyone else throw fruit at them. It will suck for the metagamer, but everyone else will have a hilarious time and it should serve as a strong dissuasion to everyone after that. 😂

Keep a bag of marshmellows nearby. Every time a player metagames throw a marshmellow at their head and tell them if they eat it their next roll is with disadvantage no matter what the roll is. 

Controversial opinion: stop caring about metagaming.

Really.  Does it hurt the players' enjoyment of the story that badly if people reference events they weren't directly there for?  Most decent players police themselves well enough on matters of truly egregious frogbuggery, and otherwise forcing your rogue/ranger to meticulously repeat everything the DM told him during his twenty-minute scouting foray before the rest of the table is allowed to know what he knows is horseshit.

Just let metagaming go.  You'll find it's not nearly as big an issue as you think it is.

Quote from dragonlord2488 >>

Keep a bag of marshmellows nearby. Every time a player metagames throw a marshmellow at their head and tell them if they eat it their next roll is with disadvantage no matter what the roll is. 

 But if they make it to the end of the session without metagaming again or eating it, give them a second one

Yeah, what Yurei said. When I play and I notice other people starting to metagame I just try to gently change to subject or go with IC knowledge anyways. If I DM and I notice obvious metagaming I also try to encourage roleplaying rather than "punishing" players. Also at most times the settings I DM in are different enough from "standard" D&D that meta knowledge often is useless. 

However, if anyone wants a creative way to discourage metagaming I'd suggest looking at the rules for paradox in the old Mage: the Ascension game. Basically, the "normal" reality doesn't like people using weird magic so if people use magic that is too obviously not "real" (like conjuring a minigun from your back pocket while wearing nothing but speedos) you run the risk of gaining paradox. Too much paradox and reality itself will try to correct you, usually by dropping literal safes on your head or having your guts torn out by a meta-dimensional dragon. Now obviously, in D&D there really isn't such a thing as "too weird" but you could play around with the concept. Maybe if the players use too much OOC knowledge of monsters NPCs will assume they are experts on the subject and start harassing them for information? Maybe people start to think they're spies or devil worshippers (how else would they know that this monster has "resistance to bludgeoning damage, whatever that is?")? In short, weird it up in ways that can be annoying but also entertaining. :)

For the people talking when characters are in other rooms, let it be in game. 

"As you yell your plans to Dave in the other room you hear footsteps and confused grunts coming from ..."

Quote from EricHVela >>

If you are okay with metagaming, feel free to throw in an idea as if you wanted to discourage it just for fun.

Every time someone metagames, they have to hold a handful of potato salad in their dominant hand for the rest of the night and can only use their offhand for rolling and note taking.

Fair enough.  Missed that clause.  If I were to discourage metagaming in a 'fun' way?  Probably Ethereal Plane pigeons that show up to peck a few hit points off of anyone who does so.  Pigeons only they can see, and only when those pigeons are assaulting them directly.

If a player is blurting out information on an ongoing basis that their character (or the group's characters) may not know, we have them roll an appropriate check with disadvantage. If they fail the check, the DM gives the group some misinformation to replace the meta gaming information. 

Examples of Misinformation:

• This creature only attacks when it's hungry. If you give it your rations it should calm down.
• The person you're talking to is actually a drag queen impersonating the person you are looking for.
• The only way to kill a troll is to cut off all of its limbs.

First recognize what the problem metagaming is.  Knowing general rules is not an issue.  The issues are:

• Knowing facts about this encounter
• Knowing facts about particular monsters

The solution is simple. a) never use monster names unless the players have battled them before.   Describe them instead.  And

b)  Every encounter, roll a d4.  Even means leave the encounter/monster alone.  Odd means change it up.  The poison trap in the chest?  Now it's a drop away floor in front of the door.   The Blue dragon?   It has a COLD attack and is immune to cold, not electricity.

I myself favor making changes to existing monsters, both in stats and their description.  I want *desperately* to run a Curse of Strahd campaign, but with monsters modified to resemble those from Darkest Dungeon; I mean, their take on vampirism is the grossest, coolest one that I've seen in a while:

Just like changing key details of the adventure

Exactly.  For anyone who might be familiar, the High Rollers D&D group on youtube has been running a Curse of Strahd campaign that has been heavily modified so that a player who has previously DM'd could play along and use some of their previous knowledge, while still having a fresh experience (of course, he promptly forgot important details that *weren't* changed, sooo...)

"Who said zombies were low level?" 

Yes, send them up against a horde of Lich's.     Best way to scare them and then confuse the crap out of them.

Quote from Mog_Dracov >>

Yes, send them up against a horde of Lich's.     Best way to scare them and then confuse the crap out of them.

 

 😂😂😂😂

One of the issues is trying to actually define what's metagaming and what isn't.  The RAW tend to have silly levels of restriction on what a character knows about a given creature- I've had GMs who would accuse a player of metagaming for having their dwarven ranger with favored enemy: giants know that you need to use fire or acid to kill a troll.  Reasonably, that should be something that pretty much everyone who lives in a world with trolls should know, unless trolls are incredibly rare.

Quote from 6thLyranGuard >>

One of the issues is trying to actually define what's metagaming and what isn't.  The RAW tend to have silly levels of restriction on what a character knows about a given creature- I've had GMs who would accuse a player of metagaming for having their dwarven ranger with favored enemy: giants know that you need to use fire or acid to kill a troll.  Reasonably, that should be something that pretty much everyone who lives in a world with trolls should know, unless trolls are incredibly rare.

 That particular instance would be aggravating. Trolls being giants, the Ranger would know everything about them.