So I had a recent game that produced an effect in Dungeon World that I love about the system. I used a Mimic to give the players an interesting encounter. The evil dwarves were using it to loot the elf town. It was in the treasury, eating gold and gemstones to take back to its masters. In this case, the Mimic was more of a construct than an aberration. The Barbarian and Psionic had a real tough time with it, though the Barbarian was currently in am uncontrollable werewolf transformation, so she wasn’t getting to do much. The Psionic failed a roll the first time to use her psychic powers on it, so that established it was immune to mental powers. She levitated it after, but we had said it was extremely heavy with gold so it was more like an air hockey puck than a floating helpless thing. It was funny and scary at the same time.
I love the way Dungeon World makes this kind of encounter seamless. You put down your normal perspective of a creature when it gets mentioned, but then it turns into something more interesting through failed rolls and whatnot. It can turn a normal monster into a scary boss in the blink of an eye.
Has this happened in any of your games?