Citron Clown Goby. Beautiful fish but a real pain. Perched on acros kept them irritated to the point where they wouldn't extend polyps. Caught it and transferred it to a tank with no sps. It then started landing on and irritating my Derasa clam to the point where is stayed semi closed up half the day. Would even peck at the mantle from time to time. Caught and returned to the lfs, just gave it back.
Also dwarf angelfish. Have had mixed results with fish that started out fine but as the got older started pecking at acros. Had a Lamarck's that had never touched anything decide that it liked tenuis all of a sudden. Cost me dearly before I was able to catch him. Just not worth the risk imho.
Purple Tang. This guy was a real terror, but I believe more my choice of tank mates than the fish.
Interestingly I have a tank with many of the repeaters. I have a Kole Tang that pals around with my Magnificent Foxface, both are the largest fish in the tank and give no grief to the others. I have a pair of bonded Lightning Maroon clowns that keep to themselves (no anemone in the tank). I have a Six Line Wrasse that's almost 2" long that's no trouble. A Neon Dottyback that is an awesome fish, gives no one any grief. He'll swim to the glass and stare eye to eye with me for minutes on end. Other fish Fathead Anthias, Green Chromis, mated pair of Bangaii Cardinalfish, 2 1/2" Ocellate Damsel. Tank is a 220g to LOTS of live rock and hiding places so I think my success with these fish may be related to everyone having a spot they don't feel the need to compete for. I also feed 3-4 times daily so everyone stays fat and happy.
As for the Firefish and Mandarine, I have a 35g with 3 Firefish, 1 Green Mandarin and a Tailspot Blenny that all get along great. All very docile fish. The 35 is tied into a 2 year old system with loads of pods so the Mandarin stays fat and happy (also will eat Can-O-Cyclops that I feed daily).