Just the usual "Oh gawd Urist, please PLEASE pull the lever! Please!" Situation, where Urist walks up to the lever.. turns around and gets a drink... heads back to the lever... goes and gets a mushroom to snack on... heads back to the lever, and feels sleepy. (Obviously from eating mushrooms and drinking hard river spirits) takes a nice long nap. Wakes up, decides it's too early to be pulling levers, and just decides to rejoice in the tortured screams of his supposed friends as he takes his clearly much more important morning break.
Meanwhile, a forgotten beast claws its way through the fortress, while the desperate overseer tries to get anyone, anyone at all to close the airlock, while jamming the hallway with as many useless puppies, kitties, chickens, and other misc lifestock animals as possible to impede the FB's pathing, while it sprays cloying and festering filth everywhere, while simultaneously suspending Urist McClothier's industrious insistence that he go down the hallway to collect webs, and Urist McHunter's incessant attempts to go hunt the native draltha.
Ultimately resulting in a tantrum spiral as Urist McCatlady goes positively apeshit from her precious babies being thrown at the nameless horror from the depths, Urist McHunter, who was on this Obsidian's issue of "Life" magazine as the fortress's most popular title holder, ends up a festering puddle of rotting flesh on the floor, causing Urist McLeverPuller to go straight from lazy asshole to tantruming douchebag before even finishing his break, while the rest of the fortress goes bonkers.
Yes Urist. You should have pulled the lever, and listened to the screaming in the hallway. Yes Urist, YOU are responsible. Everyone else was busy trying to be safely isolated against the creeping chaos, and you were standing in a corner with Playdwarf and your trousers down. Don't try to deny it.
(This exact situation has happened a surprising number of times....)