One of the users affected by the bug fixed in v5.3 has just staked. This likely caused some of the v5.2 Gridcoin wallets to fork.
If you are still running v5.2 or earlier, you should upgrade immediately. If you are reading this multiple days after this was published and haven't upgrade yet, you may need to do more after upgrading to get yourself off of the fork. You may need to either sync from zero1 or use a snapshot (File -> Snapshot Download) after updating
The reason for this fork is that some of these old wallets will incorrectly think this user's block was invalid when it was not. This issue is present in some of the wallet running the 5.2 releases, but is not in any wallets on the 5.3.0.0 release.
It is worth noting that this will not have large impacts on the network as a whole because only around 2% of the last 1000 blocks were made by users running v5.2. Additionally, not all of those 2% of wallets will have forked either.
For those curious, block 220913 is the block that may cause issues
1 You can sync from zero by shutting down the wallet and going to your data directory and deleting only blk001.dat, blk002.dat, the accrual folder, and the txleveldb folder