The other problem I've encountered on mobile devices (and even occasionally on a PC with a mouse) is that the voting arrows are small and quite close together,

The other problem I've encountered on mobile devices (and even occasionally on a PC with a mouse) is that the voting arrows are small and quite close together, making it rather easy to hit the wrong one. One possible solution would be to have only the arrow you click disappear, so you could always hit the other one and at least cancel out your own vote if you made a mistake.