According to Instapaper creator Marco Arment, some Retina iPad Mini units are having image retention issues. He observed the problems by using a grid test that he developed to detect the same issue in his Retina MacBook Pro device. He said the following:
The image retention problem was first cited as a potential issue last week. Back then, we shared with you the rumor that the delay in iPad Mini 2â€™s release is reportedly caused by burn-in issues on Sharp-made LCD panels. After Marco Arment discovered the problem, several other iPad Mini 2 owners shared on Twitter similar problems. Could Sharp be blamed for this once more?
Source: Marco.org, MacRumors