about a week ago i had a smaller reward than it probably should:
my shares: 28 total: 470,841 reward: 0.00246757 BTC block http://blockexplorer.com/block/00000000000004c59c645e133dc783c9efcc355165487b46b24cfe625f79b4f5
but 50 * (28 / 470 841) = 0.00297340291 so just 5/6 paid out

it's not much, but if it sums up...