The KimotoGravityWell retargeting algorithm is vulnerable to a timewarp attack. It's effectively a 51% attack with less than 51% of hashpower.
To fix the vulnerability a new MANADATORY Upgrade has been released. At block 643,808 the KWG fix will be activated. It is estimated to be this Friday 14 May, noon GMT.
Please upgrade ASAP and help us spread this news about the mandatory upgrade. It is important that everyone (Exchanges, Mining Pools, Payment Systems) upgrades to fight the timewarp attack. Thank you for your help and have a nice day.