I lost a block due to this. I have a python scripts listen on major mining pools' stratum connection, this morning the script heard ghash.io found a new block from their stratum mining.notify, and from the stratum connection the script immediately decoded the height, previousblockhash and bits that needed for mining the next block and send out notification to its own miners. Only after a couple of seconds, someone submitted the next block. However, local bitcoind had not receive that previous block from ghash.io. The submitblock request was then quietly ignored by local bitcoind. And even after a few seconds it got the previous block, the broadcast of the later block never happened. I lost $20k due to this.