When the difficulty falls below the previously imposed floor, a proper blockchain reorganization cannot occur because the floor is still imposed in that circumstance. Therefore, an immediate mandatory upgrade (version 0.5.1) should be installed. Not installing it will isolate you from the rest of the network.
This upgrade will be available momentarily. The fix has already been distributed enough that there is a consensus chain. Therefore, there is no risk of double spends or lack of consensus regarding the correct version of the ledger.
I wanted to clarify that we ought to raise the difficulty above the floor of 0.00024414. When we do that, clients that haven’t upgraded will work fine. So throw all the minting power you can at the network right now please.
Clients not upgraded will begin working again when the difficulty rises above 0.00024414, which is likely to occur in less than 24 hours. Be warned that if you don’t upgrade you will once again be isolated from the network when the difficulty drops below 0.00024414, which is almost certain to happen at some later date.
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.
The difficulty is currently 0.00024587, just above the old floor of 0.00024414. As expected, all 0.5.0 clients joined the 0.5.1 clients as we crossed that threshold. Everyone using either 0.5.0 or 0.5.1 is now on the same chain. I expect that to continue as long as the difficulty is above 0.00024414. So, you still need to upgrade to 0.5.1 if you have not already done so, because the difficulty is likely to fall below 0.00024414 at some point in the future.