After ten minutes or so of displaying the same image (e.g. your browser UI), a faint (but very noticeable, even readable if you had displayed text) afterimage remains on the screen for a few minutes. It's visible on any dark gray background and even just dark backgrounds in general.

This problem frequently shows up with MBPr screens from LG (I'm not aware of any problems with Samsung screens, which is the other supplier). It may take a few days of use until the problems show up and it may also be heat related (so machines that have been running for some time are maybe more likely to exhibit the problem).

I'm at the moment letting Apple replace the second screen, if the problem remains I will withdraw from the contract of sale.