There have been a number of discussions about "pity" or "sympathy" upvotes.
http://meta.stackexchange.com/search?q=pity+sympathy+upvotehttps://meta.stackexchange.com/search?q=pity+sympathy+upvote
That is, the idea that once a post is voted down to -1, some kind-hearted user will come by and upvote the post, no matter how terrible or wrong it may be, to return it back to zero.
Does the actual voting data actually support this theory -- or is it all in peoples' heads?