I had an experience with a flounder that I purchased when I first started in the hobby. The first thing it did was try to eat a cleaner wrasse. I then quickly fed the flounder and entire shrimp, about 1/2" long. Well by then his stomach was bulging. The flounder immediately went back to get the wrasse and try to eat it. He never made it because back then I did not have all the rock I have now... I got him out within 15 minutes of introducing him to the tank, and took it back to the LFS.
When I bought it I was told "very docile"... I said to the guy..."I will only explain this once, not docile at all"
Without question they took it back, and I've been shopping there ever since...!