Possible Duplicate:
Minor layout problem with negative votes
Currently, the Stack Exchange server uses Hyphen-Minus (U+002D) as the negative sign. However, in many fonts, this character is designed as a hyphen, and as a result, the bar is placed too low for the character to be used as minus sign. To represent negative numbers, it is suitable to use Minus Sign (U+2212).
I am not sure if this should be called a bug or not, but this is a minor aesthetic issue.
Screenshot of cstheory.stackexchange.com (left) and its modified version (right):