someone mentioned jellyfish - that's right - some people keep them, but in dedicated tanks w/ very specific filtration properties (designed to limit current and ensure safety around lift tubes etc.).
Here are some more. I was too tired after that post earlier to add these so here are some more:
Bumble bee Grouper- also grow to an astounding 6 feet long and best left in the ocean.
I've compiled a list that of fish that should not be kept by beginners and the reasons why, so here goes:
Panther Grouper- look adorable as babies but soon grow to a 2 foot unattractive monster. (their heads stop growing and their bodie keep growing)
Feel free to add more but this is a general list and does not include all. Research should always be done before buying a fish. (Orion-sticky please)
What about lemonpeel angels? And flashlight fish?