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...)

"Only Bad Coders Code Badly In Perl" (OBC2BIP)