Internet Explorer 9:IE 7 standards accidental change

One of our testers reported, that many pages are not working correctly in her IE9 browser.
It took me a while to find out that in development tools it was set Document Mode:IE7 standards.
The option has a shortcut -Alt-7, but it works only if development tools(F12) are opened.

I couldn’t understand, how the setting was changed accidentally.If it’s possible, many IE users will have unexpected problems.