I saw what looked like a good answer, and immediately upvoted it.
Then, as a good citizen, I decided to reserve my right to vote until I'd ensured that the answer was valid. So I immediately withdrew my vote. This worked fine. At this point in time I had voted in neither direction.
After following the links in the answer and surveying the other answers provided, I decided that it was indeed a good answer, and I tried to vote it up.
But now I was told I couldn't upvote it, because that would be changing my vote. But I hadn't voted yet.
Hey I know it's a minor case... but it punished good behaviour. So I thought I ought to lodge it.
(similar to, but not a duplicate of, question 6250)