It has probably not escaped anyone interested in Steem mining that the [miner witness queue](https://steemd.com/witnesses) has been completely dominated by one user recently.

http://i.imgur.com/ndDo4Bb.png

A lot of people speculated that it was due to GPU mining but it was later revealed that it was possible to game the queue using some clever hacks. Since only 1 of 21 blocks are produced by miner witnesses the blockchain can't be compromised using this exploit but it does make it almost impossible for other miners to submit a POW.

It didn't take long for the core devs to respond with a hard fork which should fix the issue: https://github.com/steemit/steem/issues/256

Happy mining!