I had a young plecostomus jump out of the tiniest possible gap between the back of the light hood and the filter, bounce off a wall, and then fall behind the tank....just as I was turning the lights off to go to sleep.
I walked over, picked the carpet fuzz off it, then put it in a bowl of tankwater to get any other dirt off before putting it back in the main tank. It swam down to its nook under the driftwood and never did it again.
Ah, and clown knives are very, very devious,
FYI.