I’m just wondering from some other dm’s about times the party absolutely destroyed your encounters and what you learned from 

As a recent example, the party are facing a horde of yeti, and in their second assault, the yeti had build fortifications from old furniture to defend against them. The party unleashed 2 fireballs into the "huddled behind the barricades" yeti, and cakewalked the encounter.

What did I learn: It makes sense for some enemies to be ill prepared for a fighting party. There was no reason for them to spread out before they knew that fireballs existed, and the party loved it (the wizard in particular) because they got to be effective.

The next time they faced yeti, the party had tricks lined up - fire to make them weaker, and they once again cakewalked the encounter (though one got thrown down the mountain slope, which took some damage onto them).

This time I noticed they seemed to be a little underwhelmed, so I learnt there that the novelty of a poorly prepared enemy is a fleeting thing, so the next time they fight the Yeti, they are going to have learnt a little on how to not die so quickly!

I had built a mini boss for the party, called the legion sentinel, which was a large construct with a huge blast shield that could unleash waves of force, and to add a small layer of difficulty I added a single little minion robot to trip the party up. I realized my mistake soon after the battle began- what's the point of having a huge shield if everyone can get around it? they just flanked the sentinel, one-shot the minion and won the fight in like two turn cycles. I learned that environment can make or break any monster you throw at the party, and that if you're gonna bother adding minions, make sure there's more than one.

About half my encounters get ruined. In the campaign I run, one of the players plays as a Wendigo. Because one of the species abilities was to strong, we made them have to eat a certain amount each day. So far, instead of eating deer and stuff, they have eaten a gnome after belly flopping its house, a random grandma, 2 dwarves, and one of their party members. They will spend an entire session waiting for something to return from their house just to eat it, therefore splitting up the party, which makes it just that much more annoying to run.

Quote from RoyalWerefox >>

About half my encounters get ruined. In the campaign I run, one of the players plays as a Wendigo. Because one of the species abilities was to strong, we made them have to eat a certain amount each day. So far, instead of eating deer and stuff, they have eaten a gnome after belly flopping its house, a random grandma, 2 dwarves, and one of their party members. They will spend an entire session waiting for something to return from their house just to eat it, therefore splitting up the party, which makes it just that much more annoying to run.

 
Have you tried talking to your players, specifically the one playing a Wendigo? If it's less fun and more difficult for you to run, that's not going to be sustainable long-term and the campaign will likely fizzle out in time.

I created a balanced encounter that was a party of seven level 5 players and Tiamat very nerfed vs. the aspect of Bahamut and the players just all did their own things letting Tiamat do all the work and when they realized they needed to fight or they would all die, still acted lame. This is the very bad group of players I mentioned in many comments. Also, every balanced and sane encounter gets ruined by the two players who minimax their characters, (which I can't blame them for because I minimax most of my characters) who both deal like two billion damage per turn while the other players act lame and have side conversations. Also, one time, I had the players fight some displacer beasts because I was feeling tired and didn't feel like running the next section of the game. Unfortunately, one of my players (who acts like a special snowflake because we are friends and has a tiring sense of humor) didn't like this because "there was a friendly displacer beast in the first D&D campaign we ever played blah blah blah..." and tried to befriend the displacer beasts trying to kill the players derailing the whole encounter. I like displacer beasts, but I wouldn't derail an encounter, much less an entire session over it. Those are some of my experiences.

Encounters, as a rule don't 'get ruined'.

Players certainly might circumvent the encounters I'd prepared, or negotiate their way out of it, or even just be incredibly clever in completing the encounter more quickly than anticipated. The thing is, those are all awesome things. I feel like GMs need to not phrase things in terms of players 'destroying' encounters. It reads too much like the game is then player vs GM and not setting up a world of challenges and obstacles for the players to interact with.

If my players feel like they had a real challenge but survived it due to clever tactics, that's a win. An unsuccessful encounter is where either I've made the encounter far too challenging, or where it's been an encounter without good in world reason. As a rule of thumb, if a player character has fallen unconscious, then the enemies should have lost a percentage of hit points equal to or higher than 100% divided by number of players (i.e. in a four player game the enemies should be at least 25% down on their HP by the time the first player character drops unconscious, less than this and likely I've made an impossible challenge.)

The thing I'd encourage learning it to reframe it in your mind.

As a side note here for KindaTiamat, D&D as a game is not a balanced one. It is set up so that more than 50% of the time the player characters' actions will be successful. The odds are actually somewhere in the region of 54-63% best I can figure the maths from my experience. 'Balance' is a mislead, it's an illusion that DMs at the start of their journeys often make. (Incidentally, if you have what you perceive as 'bad players' walk away from the table. I guarantee it won't be them bad, you good, it'll be a difference in personalities and play styles. Not every DM can run a successful game for every table, not every player is suited to every DM).

Fair, I need to lay on my couch and think about this. I try and be a good DM and see the beat in the players, but they talk too much and act like special snowflakes. Also, I don't know when the last time I got a "Thank You" from a player was. Maybe four months ago.

My most recent campaign wrapped up in December and the final quest was taken from the "Keys from the Golden Vault' book. The party had to steal the Book of Vile Darkness from an Efreeti. I had encounters planned, there were Erinyes, a Young Red Dragon, and some guards in the first encounter. What happens? The halfing Bard sneaks up to the dragon, uses an Immovable Rod on the tail to pin him in place, and like Bilbo and Smaug, strikes up a conversation. He eventually made a deal to free the dragon if he killed all the guards and promised the dragon could have whatever treasure they steal from the Efreeti. The Bard rolled well and succeeded. Eventually they got the book, the Efreeti showed up for the big fight but they teleport away. I threw one last obstacle at them, the Arcanaloth at the Efreeti's lair shows up and summoned two Nycaloths. What does the party do? The halfing bard runs under the Arcanaloth and crouches behind it, the rogue does an Acrobatics check and pushes the Aracanaloth back, who trips over the bard and falls into a lava river, dismissing the summoned Nycaloths. The epic finale of the campaign, not one single combat encounter.

While "ruined" might be too strong a word, one of my players had circumvented an encounter in a really funny way.

The party was enlisted by the mafia to rescue a spy from a coastal prison. The Prison itself was a stone tower, with its prisoners in the top floor, and had several guards maintaining the place. The party wizard had the entire party arrested, so they could break out from inside the jail. 

Inside the jail, the party was locked in cells, and across from old friends, enemies, and future enemies. The wizard had anti-magic handcuffs on, so he couldn't use magic. While I don't exactly remember how the party escaped, I know that the wizard had teleported himself and the spy outside, while the barbarian had broken out using other means. That left the fighter and Rouge in jail.

The prison had a special tactic to keep inmates in jail, as it would have a mummy shamble outside of the cell and use dreadful glare on the prisoners, keeping them cowed and timid. When the fighter and Rouge tried to escape, it stood in their way. The two would have to fight a powerful undead, the menace of the coastal jail, a wicked thing...

The fighter pushed it and the two ran out of the prison while it was on the floor.

I was running Waterdeep, and I buffed the nimblewright since the party was level 7. 2nd round in combat, the sorcerer casts Fireball AND Blight (which they used as a bonus action thanks to Quickened Spell), while maintaining concentration on a twin-spelled Haste. It did 100 damage, one-tapping the already slightly damaged nimblewright. 

I was going to throw a Barbed Devil at them, but they were wounded and not everyone was there, so I downgraded to a bearded devil, and it proceeded to miss all six of its attacks in the combat. I remember it as one of my worst climactic combats ever.

Quote from TiamatCultist5 >>

Fair, I need to lay on my couch and think about this. I try and be a good DM and see the beat in the players, but they talk too much and act like special snowflakes. Also, I don't know when the last time I got a "Thank You" from a player was. Maybe four months ago.

Wow .. you live in a different world from me. Personally, I and pretty much everyone I play with, thanks the DM after every session since they invested their time and effort creating/preparing an adventure and role playing everything else in the world so that the players can role play their characters. The game doesn't exist without the DM and I like to let the DM know that their efforts are appreciated. Similarly, when I DM, pretty much everyone also says thanks - even when running at a local game store for folks I might not know. So, I feel bad for you missing out on that camaraderie. 

Also, I agree 100% with Martin, there are no "destroyed" encounters. Every encounter is a different experience and I think both the DM and players get tired if every one is a nail biter since it is inherently unrealistic that every opponent faced by a party should be perfectly equal to the powers available to the party ...  and if the encounters are all THAT close then the players should lose every second encounter or so - which, in my experience, very few players would find to be fun.

 However, :) ...

Quote from martintheactor >>

It is set up so that more than 50% of the time the player characters' actions will be successful. The odds are actually somewhere in the region of 54-63% best I can figure the maths from my experience. 'Balance' is a mislead, it's an illusion that DMs at the start of their journeys often make. 

 ... there isn't any fixed "balance" to D&D at all. Since all the parameters of every encounter are in the hands of the DM ... they can make them as hard or as easy as they want them to be. The success of player actions depends on the player imagination but mechanically on the DC and skill selection that the DM assigns to the task. If the DM assigns a 10, 15, or 20 DC, the difficulty of the task goes up, chance of success goes down, depending on the skill and stat in use for the specific character taking the action. 

As a player or DM, I run into this a lot, a player of a character with low charisma role plays persuading or deceiving an NPC - it would be better for the party for a different character to make the check but it was this character that but the idea into effect - they make the check and the DM decides the DC for that character in this circumstance which ultimately determines the chance of success. If a DM finds that characters succeed 54-63% of the time, then that is because the DM is very good at reliably assigning tasks/DCs/ problems that the specific players solve with that level of frequency. I don't find it to be a game mechanic .. it is up to the DM.

However, the real question, what level of success keeps the party happy, interested and enjoying the game and their character? 90% is likely too high since it would feel like little challenge while a 25% success rate might have the players wondering how useless their characters are and why they chose to adventure in the first place. So I could see a 2/3 (66% success rate) being a range of success that might appeal to many players ... hard but not impossible and leaving a feeling of accomplishment when the party succeeds,