I've just taken delivery of a replacement 27" 2013 iMac the first one had a yellow tint at the bottom of the screen and this one appears to have image retention. It is not visible on the normal desktop during normal use, but it is visible, as a faint outline of the windows on a previous screen, when I swap to the grey chequered background of the widget screen. It disappears after a while, but my concern is that this problem will increase over time. I'm still within the two week return period, but I don't want to return another iMac if this is normal behaviour for 2013 iMacs or IPS screens in general. I hope that someone can assist.