After the Big Fight, the PCs needed a long rest and so they locked themselves in a room. They thought the whole dungeon was cleared except the one room marked DANGER and locked from the outside. They forgot about one goblin they had seen lock himself in the pantry and did not know about another.
While they rested, one goblin slipped out of the dungeon, after opening the DANGER door and letting the zombie out (which the PCs had to fight after their rest). The other goblin tried to get to the (now dead by PC) boss' treasure chest -- which was trapped. That is how they found the treasure chest, with a dead goblin twisted by rigor in front of it.
The finding the trap and examining the corpse was all fun and engaging. But the climax of popping off the trap from across the room was boring and, like I said, since mage hand is a cantrip it did not even cost the PCs anything to do it. Boring.
If I were to rewrite that trap and wanted it to matter, I think I would make opening the chest drawer caused the door to the room to slam closed and lock, and the poison gas would be enough to fill the room (rather than just the immediate space of the chest). But then, like I said, I like big complex traps that present REAL danger.