Had these both happen today actually...
temperature.
My roommate, who is known for her clumsiness was doing the h20 changes and bumped into the bucket's handle. Little did she know, she'd sent one of the 2 inch fish spiralling 4 feet to the ground (big fall for a little girl). When I came back 10 minutes later to release the fish I noticed one missing. I checked the tank but found nothing and then my roomie spotted her on the floor behind the tank. I thought she was dead for sure so we got some tongs and I gently went to pick her up. She flopped so I quickly lifted her back to the tank. After getting rid of the dog-fur coat she'd acquired on her vacation from water she spent a few minutes in shock I think. The barbs in the tank nipped at her a bit and she barely responded. Fifteen minutes later she seemed almost back to normal. We'll see how things go.