[cfaubell]

The number of times I've had to fix someone's "broken" browser because they accidentally pressed F11 is depressingly large.

[hexagonc]

The problem here is the lack of communication. How hard is it to show a brief popup telling the user what is happening and how to reverse it? At lot of applications with a fullscreen mode already do this. For example, Parallels and VMWare Workstation do it[1]. Options that can be confusing or dangerous should come with warnings instead of just taking them away.

[1]I think there is a way to stop showing the message, though.

[singleuse]

Opera does it and offers an opera:config checkbox to turn the warning off.

Firefox is one the few who does not offer this feature.

[mkr-hn]

Chrome does that when you hit F11.