I wrote a poll section for the company i work for, on www.sgl.net.au.
I was testing boundary and illegal values with my script, and i set the number of votes for one of the options to
-1, when i click "Vote" it takes the -1 and increments it. (taken from a mysql_db) when it finishes incrementing it the resulting vote number is -2

so 
$variable = - 1;
$variable++;


echo $variable prints out -2
The problem could be with mysql returning a positve integer only ,but i thought i'd mention it anyway