This
keystroke is basically the generic
macintosh command for
NO. STOP THAT NOW.. It takes the place that
control-c/
abort/
break/
esc holds in most other OSes. Once upon a time the control-. hotkey was simply universal; now it's just common, but still works all over the place.
Command-. will automatically stop getting your email, or copying files, or most any progress box. It will almost always automatically hit cancel or stop in any active dialog box. It is the only way to make the "Insert this disk" modal dialog box go away.
And in an interesting little half-finished feature that appears to have never quite been implemented fully, it is possible to use command-. to cancel the launching of an application.. sort of. Just as the zoomrects brush the menubar and disappear, just as the menubar goes blank.. in the instant before the application's name is drawn in the menubar, there is a fraction of a fraction of an instant wherein if you can hit command-., the launching will abort and the zoomrects will actually shrink back down into the application icon. You will never actually get this to work when you need it to, but it's fun to play with, opening and closing some small app over and over trying to hit the tiny hidden trigger.
Watch an old-timer mac user for long enough, and you'll eventually notice that if they ever screw something up bad, ever give some command to the computer they didn't mean to, they will close their eyes and start simply banging command-period over and over for a couple seconds. They do this even if they're in an app they're fairly certain doesn't support command-period. They can't help themselves. It's too deeply ingrained.