I had this happen with a Lubbocks that was with me for over a year. He was perfectly fine for that year -- he even survived jumping from the tank once when it was open for me to work on it, because he jumped into a bucket of saltwater! But one day, when he hadn't appeared over an hour after the lights had come on, I knew something was the matter. I looked for him everywhere, and it was hours before I found him wedged between some rock and the tank wall. My best guess is that something spooked him and he wedged himself so tightly that he could not get free and died. Had he done this somewhere else in the tank (instead of against the tank wall), I never would have found him before a clean up crew took care of his remains. It was very upsetting, and I feel for anyone who has lost fish like this.