I'm trying to find the discussion about this. It seems that a couple of miners are monitoring activity to see if a block has been partially completed, and then jumping in with a partially populated block to grab the reward before the finder can complete. Now I'm posting this from memory, and I don't understand the stages that miners go through to prove their blocks. Also, I gather that it's only careless miners that leave themselves open to this. If anybody recvognises this description, I'd be grateful for some keywords, so that I can find the article again.
I'm struggling to find this. I wonder if it was something in the discussions about "selfish mining"?
If you mean by partially completed block a one that has not been broadcast,
then that is impossible.. and no miner would broadcast a block that fails to meet
the PoW target.
There is a scenario where a miner can take a valid, broadcast block, stuff in it
some new transactions and start to mine on it, and if he succeeds, he effectively
"hijacks" the reward plus some extra fees. I don't know if that actually occurs, but it could
be seen as a variant of selfish mining (where a miner finds a valid block but does not
broadcast it, instead starts to mine on top of it)
