1. A couple of female bettas.. it was early in my tank experience and I admit I did not know better. The girl I 'rescued' my first tank from had 1 male betta in with 2 female bettas in a community tank... when the 2 girls passed away (they were over 2 years old) I got 2 more females, trusting in her advice that that would be fine. Of course, things went bad and I ended up having to get more tanks to accomodate the female bettas..
I suppose that is a lose/win situation, since it was the start of me adding more tanks to my repertoire.. but.. I do sometimes wish that my tank additions allowed me more freedom to choose other types of fish (so many do not mix with bettas).. I would LOVE to try chichlids.. but.. then again, I would not trade my girls for the world, so it's a confusing mistake..
2. Red-Wag Platys... I bought 3 and they died with in a week.. nooo idea why, since everyone else in my 3-year cycled tank (a betta, zebra danios, ADF's and cories) was healthy...