Something like the following scenario has happened to me two or three times since I've been active on this site (and happened again today).
I read through an answer, like it, and upvote it. Then I take a closer look and realize I'm not so sure the answer is correct. I remove my upvote, which the system allows through (I believe) five minutes. Then, after thinking through what bothered me about the answer, I realize the answer actually is correct and click to restore my upvote. However, instead of recording my restored upvote, the system gives me this message:
You last voted on this answer
[x time units] ago
Your vote is now locked in
unless this answer is edited
The net effect is that I now cannot upvote an answer that I think deserves it.
I don't know if the same problem occurs with downvotes; I've only experienced it with upvoting.
This sure looks like a bug to me.