I'm in a fantasy point league, sandbox to be exact, and my biggest mistakes have been the following.
1. Waiving Adam Eaton for Cliff Lee at the start of the year, only to waive cliff lee a week later, i know eaton has stunk, but i really think he was worth a roster spot, 12 team league, someone else snatched him.
2. Having the brains to snag victor martinez in the 22nd round of the draft, only to waive him when he was doing mediocre at the start of the year, it wasnt a panic waive, just a casual thing to pick up another catcher, since i already had piazza it was only for a backup.
3. Sticking with herges for as long as i did, in my fantasy point league, there is 140 relief appearances, so if you use one up without a save, its kind of annoying, as you know herges is the king of non save oppurtunity appearances, and he usually stunk in them.
4. Other than that, missing out on chone figgins 3 SB 4 hit night, and daryle wards cycle(since waived), both on my roster, both benched