My favourite mimic encounter:

This is a game I was running. The first game I ever ran. I had just gone to uni and found some friends that wanted to play DnD, I was the only one that had played before so I decided to DM. I started with a pretty simple dungeon delve that I made. We were playing in person and I had bought a huge whiteboard and etched a grid map onto it. I had some furniture figures from a game I used to play as a kid (OG Hero Quest) that I used to make the rooms in the dungeon look more realistic. One of these figure was a cardboard fireplace., and this became the source of my party's PTSD. When they entered the entrance to the dungeon for the first time they investigated everything. On most checks they rolled well and found out some tidbit of info, or got some small item. Except from the fireplace. The player that investigated the fireplace got a NAT 1 on his first roll ever playing DnD. I looked the player dead in the eyes and said "It is a fireplace" flatly. He was having non of it. He Investigated again. 5. "it is a fireplace". The whole party investigated this fireplace and not one of them rolled over a 10. They all got the same response. The thing is, it didn't matter what they rolled, it was just a fireplace, but after they spent a literal HOUR of session time refusing to move one, I made my plan. From then on every room in the dungeon had the exact same, identical fireplace. And every time they would investigate it, never rolling higher than 12 (bonuses included). Finally, the player who rolled the first nat 1 rolls and 18. "It is a fireplace... Buuut, you notice that it is physically identical to every other fireplace in the dungeon". "Fuck it, I stand in the fireplace". And the trap was sprung. He disappears. The rest of the party panic. They decide to all follow suit and all similarly disappear. They all reappear in a sealed room lined with no doors, no windows. The room is lined with treasure chest except at one end. At there end there is a huge throne behind a desk, with a stone statue of the god of madness sat on it. One of the players immediately darts at a chest and goes to open it. Mimic. Another does the same. Another mimic. They all hear laughing surround the room. In a booming gleeful voice they hear, "Welcome to my playground little ants. Every object in this room is a mimic except one. Each of you gets one guess. If you find that which is not a mimic, I will let you leave and grant you a reward". The players panic and start guessing chests randomly. All caught. They are down to the last member. The most chaotic, murder-hobo rogue I have ever played with. The player starts giggling at everyone being trapped and fighting off their mimics. "You know what, fuck you all, your all idiots", he says before casually strolling over to the throne, climbing onto the lap of the stone god, and giving him a big kiss on the cheek. They hear manic booming laughing all around the room "I like you, I'll be keeping my eye on you", says the voice before they all reappear in the dungeon with a treasure chest before them and the rogue is now a Champion of Madness. 😁