I've had fish literally disappear into thin air, never to be found again. One time when I was doing water changes I had a water deionizer filling up a tank with a baby tiger shovelnose cat as the only occupant. The phone rang and I got distracted and forgot all about the fact that my tank was being filled up. When I remembered, I went to find water was leaking on the floor and the shovelnose was gone. I sware I looked all over the entire room under every shelf, behind every aquarium, in every aquarium and he never turned up. That's about the weirdest fish death thus far...