This post isn't specific to Puzzling.SE, but I am posting it here, since this is where I learned it.
I was a member on Puzzling.SE for many months before I figured out how to correctly undo an up-vote (or down-vote). The first 3-4 months I would just click down-vote to undo the up-vote, and then get a little frustrated that it wouldn't undo, but would actually down-vote.
Eventually, I accidentally figured it out by trying to click the up-vote button again, and realized that it undid the up-vote.
This should be more dummy proof, on all stack sites. Maybe a message should be there, or pop-up, to educate the user.
I suspect that other users have had the same problem.