I am about to get my Nook replaced for the second time due to this:
It looks like screen damage, but if you turn the light off the screen is perfect. This time I know exactly when it happened too. I put the Nook down on my bed for a potty break, came back a minute later, turned the light back on and this was there when it hadn't been there a minute ago. Nook wasn't dropped or handled roughly, and put down on top of a mattress.
Luckily B&N is replacing this, even though I'm a month out of my warranty period, but I think they know there is a big issue with these. I'm just concerned this is going to happen again.