…nce 950 of last 1000 blocks are mined by 0.6.0 clients.

2) Fix a couple of freezes due to locking.
3) Use last block hash in PoS mining to prevent atackers ability to premine a chain of PoS blocks at certain times.
4) Fix rare crash when accepting PoW blocks via submitblock function.

Note: Client will try to reindex blockchain automaticaly. If that fails you should try re-donwloading blockchain.