I liked the first answer on this thread. Not getting a big enough tank is a good one. Another big regret of mine was killing so many fish that I love (fresh water puffers) out of ignorance. They are what moved me from just a goldfish guy to getting into fish in a big way. It's been many years since I first had a puffer, but they helped me make the transition to cichlids etc. The problem was partly mine, but also a lack of clear information online (this was 2005/06) about how to keep puffers. At first I put them in fresh water and they died. They I put them in brackish water, but it wasn't marine salt it was aquarium salt and so again, more died.
Today I know how to keep puffers, but I'm gun shy because I lost a half dozen or so and it bugs me to this day. Another regret was keeping German Blue Rams in water that was too cool. I had them in a 78 degree tank because that's what so many websites said a year ago. Corey from Aquarium co-op seems to have made people come to their senses about how GBRs need warm water (82-86). Even my LFS now specifies on all of their tanks that GBR's need 84 degrees or higher. I lost a beautiful big guy just to sheer ignorance. I almost cried when I had to fish him out of the tank the day he died. Losing fish sucks ass.