Had a very bad screen burn in my S5 - i could actually see the title of the email app i have to use for work on a white screen without sunlight (title of the app is ALWAYS at the top when working in the app). Now had the S6 since it has been out and starting to see the same issue. Spoke with Samsung about this and they stated that the S4, S5, S6 and all Note devices are capable of getting screen burns with High Contrast imaging or lines. They said they could replace my screen but i would be without a device for nearly a month, and the carrier will not complete a warranty because the device still functions perfectly.
That's the nature of OLED technology. It's not Samsung's fault. Any OLED or plasma display will inevitably suffer from burn in.
They key is not to keep static or super bright images on the screen for extended periods of time.