I had a bunch of SB Tex babies growing out in a 55 with those tetra pitcher-type filters, where they can swim in the top, I was replacing the media one day and found a 2 in fry UNDER everything (floss, carbon bags etc) stuck, which a big gash in him. I put him in a hospital and he recovered in about a month. I think he got in there by accident though and got stuck, or one fish chased him in there.
I had a KKP fry that tried to live in a pitcher type filter that had broken off the suction cups and was hanging half way down in the tank. he was just sitting in the top of it though for cover so I removed the filter.
I have only had this happen with the pitcher-type haning in-tank filters like these
At the shop i work at a few weeks ago I found an engineer goby in a sump ACROSS THE ROOM FROM THE TANK HE WAS IN.
He must have jumped out, crawled across the ground, and somehow jumped 2 feet up into the sump tank.
I used to have clown loaches in my sump 12 of them >_< I blamed the oscar for eating them, once the tank was moved noticed clowns in the sump lol. god that was a nightmare catching them out >_<.