Not a costly mistake, but I felt bad nonetheless... When upgrading my tank, I removed a couple pieces of rock that I didn't want in the new tank and put them outside. About 45 minutes later, my wife asked if I transferred the porcelain crab... There it was, upside down on the ground and a completely different color. She threw him in the tank and nudged at him. In about an hour he had made a full recovery. How'd he stay out of the water for 45 minutes!?!?! Still alive.
Next time I'll be sure to do a better head-count.