We use to get tiny live baby crabs from shucking oysters. This was before I was into fishkeeping so we always fought over who got to eat the little critters.
The wildest "stowaway" I have ever experienced was on a troll just off Chesapeake Beach. I steered the troll into a massive baitfish surface break roughly the size of a football field. Needless to say every rod on the boat doubled over leaving all of us to have to work two rods at a time. We landed may small rockfish (striped bass) around 24" - 28" long range.
We were just hauling them in as quickly as possible and chucking them onto the floor of the boat as we'd bring the next rod in.
We got all the lines in finally, and darned if we don't see around a 13" bluefish flopping around on the floor of the boat. We were all so baffled, we sure a F--- did not catch any bluefish!
We were of course "maxed" out on our creel limit within the minutes the troll took, and were heading back into port. My nephew pointed out that a rockfish had just "puked" another foot long bluefish up. This one was not alive, but had obviously been eaten just prior to us landing them.
We gave the blues to some folks at the fillet station of the dock.