This question already has an answer here:
I have a strange issue. I reached my daily vote limit a while ago. I keep reading and editing and answering, and sometimes I just can't help but clicking an answer.
Typically here, my answer was wrong, and I voted up the other answer when I noticed it. I got the "daily limit" message, but the vote still got in. I may have done it elsewhere actually, since I just realized this.
Is it a browser bug? (as in, what the browser shows is not the reality)
Is it a SO bug? (as in, the database actually has now inadequate information)