My 1 year old n800 has started showing this problem yesterday:

after the screen has been off varius hours, when it turns on I get horizontal black lines running up and down the top third and the bottom third of the screen, but only if the background image is mostly dark (e.g. the unlock keypad or the home screen with an image), no lines with an app open (even looking at hackaday, which is black!) or with the home screen set to a white background.

Eventually the lines disappear completely, which makes me think it's a faulty electrolytic capacitor somewhere, but it could be anything else.

I searched and I only found people reporting vertical lines, and that issue was fixed by reflashing, but I think this is a different one.