Statistics clearly show the withhold occurred.
Slush, the oldest Bitcoin pool, clearly had no idea about checking this problem, even years after running the pool, until he was told about it by others.
Genesis tested their 'new' system on the live Slush pool and thus made the Slush miners pay for Genesis' failed testing.
Genesis also didn't bother to keep track of their testing and statistics suggests that they lost in excess of 20 blocks when blocks where 25 BTC, so in excess of 500 BTC
Slush hid these details and let them off the hook, and the rest of the miners on Slush paid for this 500BTC or so of testing.
I'd suggest you avoid Genesis (and Slush) like the plague.
Don't confuse. Just go with bitcoin pool not with slush, genesis or other...!!!