Also, Escape key won't work if you had pressed F11 to switch to a fullscreen mode. Furthermore, browsers like Opera remember this setting and restore fullscreen mode (along with the page content) even after restart. I saw a case when a user somehow has opened a page with ads in fullscreen mode (probably it was done for watching a movie) and it was restored even after exiting and restarting the browser. So I killed the browser with Task Manager, restarted it and the fullscreen ads appeared again.

I (an experienced user) couldn't quickly figure out that it was actually a fullscreen mode, couldn't understand why the browser doesn't launch and thought it was some kind of virus in the system. I figured it out only when I tried to move mouse upwards and a browser UI appeared.