The first time I tried to undo an UP vote I naturally tried to click on the DOWN arrow expecting the score to subtract 1.
Instead I had to click on the UP arrow again but logically if you are not aware of the limit/rules you would think it would iterate (+1+1=2 not 0).
Of course the same is true for undoing a DOWN vote.
Here's the proposed change: when you have voted once the opposite arrow acts as an undo.
1/ undo a UP vote: click once on the DOWN arrow (and a second time to downvote)
2/ undo a DOWN vote: click once on the UP arrow (and a second time to upvote)
And remember to change the tooltip dynamically.
"The lowest voted feature request we look at has thirty upvotes" so remember to upvote this question!