The 0.5.0 mandatory upgrade had been planned for over a month. There was nothing hurried or urgent about it. We had identified over a month ago that there was a minor protocol flaw that prevents the difficulty from dropping below 0.00024414. It was intended that the 0.5.0 release would remove this flaw. And it did, except when a blockchain reorganization occurs. In that case the floor was still being mistakenly imposed in the 0.5.0 version. The difficulty did indeed recently drop below 0.00024414. 0.5.0 clients then became unable to perform reorganizations, causing multiple forks throughout the network. The solution to achieving consensus was to remove two lines of code that separately enforced the difficulty floor in the case of a reorganization. The removal of these two lines enforcing the difficulty floor is the only difference between 0.5.0 and 0.5.1.

In summary, the portion of the network using 0.5.1 is functioning optimally and is producing more than 1 block per minute. If you are using an older version, chances are high you have forked from the longest, consensus chain. The 0.5.0 client will accept the longest chain and its immediate problems will be resolved when the difficulty rises above 0.00024414 again, which I expect to happen in less than 24 hours. However, it is still important that you upgrade to 0.5.1. If you don’t, you will be separated from the network the next time difficulty falls below 0.00024414, which is almost certain to happen at some point in the future.