I voted down a bad answer. When I came back a bit later I found that the problem that caused me to downvote had been fixed and I tried to vote up once to return it to a zero state. Strangely enough this ended up with it showing one point instead of the zero it should have.
I figured my downvote had been replaced with an upvote so I tried clicking right on the number itself in the hope that was neutral--but this resulted in it showing +2/-1. What?!
Edit: Nobody has addressed how the votes ended up +2/-1. How was I allowed to upvote twice?