One is where the backlight is flickering, like a old TV or some light-tube with bad refresh rate.
And the other is screen/image corruption mainly in the top bar next to clock, in stock browser and some other place.
I have had the 2nd, screen flicker/corruption and it seemse to be related to the notificationbar - having items like unread e-mails can make it "break" for a sek or two.
I have also had the first, screen refreshrate mainly in low light setting. It behaves better with automatic brightness but not as obvious.
this is mostly notisble when you have a dialog box with a dimmed background around it.