So here's what happened: I upvoted the answer (previously at 14) to a score of 15, with the upvote button filled in. I then locked my device. Upon waking it back up, I saw the score was at 15, but the upvote button was no longer filled in. I tried upvoting the answer again, but got this error message:
I would assume that the bug has something to do with saving the current state, but I'm not sure.