While this doesn't actually register the votes, it makes it seem like users can upvote infinite times:

From Louis Garbour on Google Play:

New bug I recently found: there is a way to infinitely upvote/downvote a comment. The following is for up voting. Just swap the up votes for down votes and vice versa for infinite down voting. 1: Upvote the comment. 2: Double tap down vote on same comment. You will notice that the comment has a score one higher (or lower if downvoting) than when you started.