I wish there was some indication if the node was zero or negative before you voted, because sometimes I casually downvote something, not realizing it had already been carpet bombed.
You could use worst nodes (the posts are rated !) to spot the 'alredy beaten hard' posts...
UPDATE : Oops ! I miss footpad comment on this point...
My advice if you feel bad : spend time to browse through the person you just downvoted's node to find a good one to ++ :-)
You might not only 'correct' your previous action, but also learn new things and even maybe 'know' the downvoted person better (and hence maybe understand him...)