I'm currently looking into some unusual results from slush's pool payouts. Most blocks on which I complete work (shares) receive the appropriate payouts however some display shares completed but no payout amount.
A screenshot of this issue can be found at https://docs.google.com/file/d/0B_G2Isi7ITVMR25OcnRBYXkwams/edit?usp=sharing .

Can anyone explain this behavior. Please don't just guess. If you don't know why and can't test your theory, please move along.

I don't think these are stale shares because the odds of every single one of hundreds of shares being stale in the same block and this issue occurring on multiple blocks seem incredibly unlikely.
I don't believe this is a case where no work was successfully submitted because I was apparently awarded shares by the pool.
I don't think this is a case of simply being rounded down to 0 because I don't meet the minimum for a payout because the division works out to more than other blocks for which I have received a payout.
These aren't simply unconfirmed blocks because there are blocks that are confirmed that also have shares but 0 payout.

However, if you can find some way to confirm any of those theories, I'd be glad to try it.