These are a few things I went through in the past. I'm in a keeper league, in our 13th season and I've had a lot of success but...
Lost a championship because I started Billy Volek (who was on fire) over Brett Favre (who always sucked @ Minnesota). Their point differential was the difference in my game's outcome.
Lost a championship because Maurice Jones-Drew went down untouched, got up and went for an 80 yard TD run. Of all the disciplined team's he was playing against, it was Belichick's Patriots D that had a brain fart. I still had a chance that game with Wes Welker (then with Miami) only needing a few points and there was a monsoon in South Florida. I can't remember if he even had a catch.