If you visit System Preferences> Displays under Mountain Lion, you may notice something odd.
The familiar “Detect Displays” button isn't there anymore. Huh. Well, that was useful, and I'm sad to see it go.
No, I'm kidding. You can get it back, in a completely non-obvious way. “Detect Displays” is handy if you use different projectors, monitors, and the like; if something seems wonky or if a display just won't work properly, this feature is a great way to force your Mac to check and adjust its settings for those devices.
So to get it back, just hold down the Option key, and voilà—that fantastic button will reappear.
If you've already got an external display connected, you may see a “Gather Windows” button in that space instead.
Similarly, if you'd like to use the “Detect Displays” feature there, just hold down the Option key, and “Gather Windows” will change to that. Cool, right? Now we just need to figure out why Apple decided to hide that ability in the first place. My typical response to being confused by Apple's choices—shaking my fist at the sky—hasn't yielded results so far.