I could reproduce this with the DemoBrowser (WebEngine) compiled with 5.6.0 release.
1 - Go to www.youtube.com
2 - Choose a video, start playing and hit the button for full screen
3 - Press Escape to exit full screen mode
Notice that you end up with a gray screen, which is hard to quit.
I've only tested it on Mac OS 10.9 for now but I will shortly try on Windows or other Mac OS versions.