Although this doesn't seem to relate to the janky LCDs that some users were seeing on their 20-inch iMacs, Apple has admitted that at least some of the newest machines do have an issue. Reportedly, an Apple spokesperson stated that a "small number of iMac users have made us aware that a recent software update can sometimes cause their iMacs to freeze, requiring them to restart." Apparently, the machines become "unusable, requiring a hard reset in order to recover," and Cupertino is currently "tracking down the root cause of the bug." So, if you just so happen to be moments away from chucking your new iMac through the nearest window over this, restrain yourself -- a software update with the cure should be coming "later this month."
AppleiMac 27-inch with Retina 5K display (late 2014)