F11 is often fullscreen, and F12 is sometimes a screenshot button in some games.

If using a debugger with Visual Studio-type keyboard shortcuts, F11 = step in, Shift+F11 = step out.

If running under the Visual Studio debugger, you can also press F12 with the debuggee in focus to break into the process.

I use F12 several dozen times per day because it is the "Save as..." shortcut in MS Office. Much better and quicker than navigating the menus

Full screen and dev tools respectively for web developers.