There are some modified versions of mining software (I believe it was cgminer) that can be set so that found blocks are not submitted to the pool. There was a miner in China earlier this year that was mining on the eligius pool, withheld what should have been several blocks (they would have been expected to find several blocks verses what they actually found); it ended up costing the pool several hundred BTC. There was also likely a similar attack on BTC guild that lasted several months before that as their luck was way below what it should have been.