This pool and Slush both found a "471190" block within seconds of each other (both building on top of the same previous 471189 block). The timing was so close that the "network" gets to decide which block is the "valid" block by waiting to see which one the next block (471191) is built on top of. If it's ours, then we get declared the valid block and theirs gets orphaned; if it's on top of Slush's, they get declared the valid block and ours gets orphaned.