Flashing tilefish. Most people probably have never seen one, but they are one of the most unique amazing fish in the hobby. They literally flash blue/pink/puple and various shades between quickly before your eyes. The change in color is very obvious.
He was about $100 and jumped out of my tank within a week.
Well, a few years later, I saw another one at my LFS. $80. However, I was prepared. I placed plastic grid (flourescent lighting grid?) over the holes in the top of my tank so he couldn't jump. Apparently there was a tiny gap between my heater and the hole above it and he managed to jump out one night. The cat alerted me to it. Luckily, he wasn't out of water long, and everything was fine. A few days later, I saw it poking its head out of the water, bumping against the grids covering the top of the tank, and repeating this many times. Within one day it managed to squeeze through one of the tiny spaces in the grid work and died.
Dwarf angels. They always end up picking on my corals.
Blue damsels. No explanation needed.
Whitley's boxfish (stupid purchase in my early reef days). Did the poison trick and killed all my fish including itself.
Darin