Geebus hadn't found the issue yet when I posted about it.
But what Geebus found is also very interesting... Out of the last 50 blocks that the pool has found, 4 of them have been invalid.
That's an 8% invalid rate over the past 50 blocks!
In the mean time (during basically the same time period), Slush's pool has only had 2 invalid blocks out of the past 500 (0.4%). And Tycho's pool pays 48.5BTC for each block found even if it is eventually found to be invalid.
If they are working on a block and there are 2 other pools that are 5x and 8x faster than them, then that wouldn't exactly surprise me. One of the other pools comes ripping through and finds the solution right before bitcoinpool doesn't seem that far fetched.
That's not how it works.
Invalid blocks are a function of how far away (from an average node distance standpoint) the pool's bitcoind is from the rest of the bitcoin network. A high number of invalid block submissions implies that the bitcoind instance is relatively closer to an edge of the network than the center.
The pool is working on a block with a prevhash pointing to the previous found block. But if the pool's bitcoind doesn't have the latest information from the blockchain, then it will be working against the wrong prevhash. The further away the pool is from the center of the network, the longer it will take (on average) for the pool to get blockchain updates.