I tend to vote such threads to around a couple of karma myself. 0 isn't unreasonable. < 0 is quite peculiar. But my confusion was resolved in this instance. Someone messaged me and explained that he was trying to work out why the votes were fluctuating so much (4 -> 0 in an hour) so was testing what would happen if he put down one more to -1.

As a side effect to these posts going up and down I've now started paying attention to only the last digit of the score. The 10s are mostly noise!