I have a late-2014 5K iMac. Just picked it up and absolutely loving it! I've noticed that against some backgrounds (grey/white, principally) you can sometimes see image retention ("ghosting"), especially in "set" areas - the menu bar (though I normally hide it) and on reminders which don't automatically dismiss along the right side. An image of it is attached - though it makes it look a lot worse than it actually is! In all cases, the imprinted image disappears within just a few minutes. I've never had an instance (in the short time I've owned it) where that hasn't been the case. I'm aware Apple has made note of this and suggests setting up a screensaver, which I've done.