I have encountered this bug twice today. How? I was reading a question, then I decide I would upvote it and click on the big Puzzling headline to go back to the main page. If the time between the two actions is too small, I see this message for a second: "An error has occurred". One of the times my upvote was not accounted so I had to redo it, but the second time the upvote was correctly counted.