It's on this last reason that I make my choice for this poll. I voted for George Bush in 2000 and regret that I did. My vote made no difference in California (where he lost by a wide margin), but it still annoys me that I "did not see it coming" when he went nuts with a lot of bad policies (bad fiscal policies, domestic spying, crony capitalism, etc.) and actions (invading Iraq, torture and suspension of habeas corpus, etc.). I am just annoyed at the thought that I associated myself with his administration. (Ironically, I've "thrown away" my vote on libertarian candidates before, so this was a conscious decision.)
At the time, I was thinking Bush II would be like Bush I, who would not have been so bad as an Al Gore who would -- in my view -- do anything to get or keep power. Gore's flexibility wasn't as appealing to me as Clinton's, taking it for granted that all politicians have some quality of flexibility.
Case-in-point, we have Gore's recent admission [read this and this] that he only supported corn ethanol because he wanted to win votes in Iowa. I wish that Mr. Inconvenient Truth has said something before we got one of the most destructive agricultural programs out there!
Sorry if seems more confessional than analytical, but I want to be clear on how these regrets work, if only with one data point. Do you have a story to share?
Bottom Line: Sometimes we make voting mistakes. Learn from them. (Corollary: Sometimes politicians make policy mistakes. Take away their power :)
Addendum: Read this great piece on the contortions of ethanol lobbyists trying to defend their $6 billion (plus!) boondoggle.