Make a vote. Click on a link on the page. Hit the back button.
At least for me (FF 3.0 on Ubuntu), I see the orange vote marker, but the number has not been changed. Often, I voted up an answer that had 0 votes, making me believe that somebody down voted it.
Of course, refreshing the page fixes it, but this is sort of annoying.
EDIT: As I figured, this seems to be a browser cache issue. I still consider this a bug as the orange vote marker is there, but not the changed number. I don't know much about web development, but it seems to me like this should be possible to fix. Any ideas why this is happening?