iCloud password refusal
Anyone using an iOS device might have noticed right away or using any iCloud enabled app on the Mac (eg, Addressbook).Twitter is full of such reports at the moment. One can reset the password but the new password gets refused as well.
I got freaked out because my Mac was beachballing while running Synk (which I had seen a handful of times in the past) and I did a forced restart. When the computer booted up again all the little apps that log into iCloud (Calendar etc.) showed that iCloud had refused my password. And since my iCloud account can wipe my HDD (Filevault + Find my Mac), if anybody hacked my account they could cause serious disruption.