Apple is acknowledging that some "Sandy Bridge"-era iMacs have issues with their graphics subsystem and is offering to repair them, according to 9to5 Mac.
The iMacs in question were manufactured between May 2011 and October 2012, when Apple introduced the radically redesigned thin iMacs that eschew internal optical drives. Specifically, 27-inch versions of those iMacs equipped with AMD Radeon HD 6970M graphics chips are prone, under certain circumstances, to failure.
Official service resources including AppleCare, Apple retail store Geniuses, and authorized Apple service providers "have been given a series of steps" to verify the issue and facilitate a repair, if necessary.
Source: 9to5 Mac