Here's a few failures
Bicolor and Lemon peel angels: neither ever really got on well with my hippo tang stopped eating and died in a week.
Fourline red sea cleaner wrasse: Fine for a few days then died mysteriously. (Tried this twice DOH!)
Here's one I haven't seen, here, yet: Sally lightfoot crab Confirmed kills neon goby, firefish, ocellaris clown
Sixline wrasse: confirmed kill on a royal gramma and almost on an ocellaris. Healthy fish, but mean, died in a power outtage
Lessons I learned the easy way
No damsels in the reef
No pseudochromis ( I did keep a P. diadema in a 20 gal with an ocellaris clown at school no problems)