I just by accident up voted an answer that my first choice was to down vote. After the down voting i pressed the up arrow so to "cancel" my down vote and that registered as an up vote. At this time i didn't noticed and i come back after a small amount of time (8 min) and i try to cancel my up vote by pressing again the up arrow but it notifies me that "after 8 min you can't cancel an up vote unless the answer is edited". I don't understand this rule, why an accidental up vote can not be cancelled?
This has already been asked on Meta.SE. Jeff Atwood himself gave this reason:
would you also like the ability to change your vote after the president has been elected? Sorry, but vote undos have historically been our #1 source of gaming and exploits. The window is very limited for a reason, so vote carefully.
Whether or not his election analogy is useful, it is pretty obvious how that can be a source of exploits.