I couldn't repro this so don't have more info, but I received "An Error has Occured" while trying to upvote an answer. What I think happened was I tried to open the comments section for the answer while it was still handling the upvote. There is no error sitting in my FF error console for it. Going back to the main list, then to the question again, I could upvote it successfully.
These are usually one time occurrences that could be anything from a random dropped connection, error in downloading, an upgrade was going on, your router hating you, anything. A refresh should fix it.