Such a mechanism would be very easy to abuse (vote to see what the node's current reputation is, then "back out", and vote according --or against-- the majority). It would create many more problems than it would solve.

It's just a vote, so don't sweat it. If you intended to vote "++" and voted "--", maybe /msg the person to explain the mistake. If it was the other way around, well... too bad.

--ZZamboni