(Bug is present in Safari 5.1.3 but not Firefox 9. I don't have access to any other browsers to test at the moment.)
If you press almost any character, the keydown and then keyup events are fired once each, as you would expect.
When you press the escape key in Safari 5.1.3, the keydown event is fired what appears to be once every single millisecond that the key was held down for and the keyup event is fired once.
Digging deeper, this looks like purely a Safari bug: http://jsfiddle.net/PLtjN/ (Ticket filed at https://bugs.webkit.org/show_bug.cgi?id=78206)
Unless there's a way to smooth over the bug with MooTools, looks like I'm stuck with workaround hacks. ;_;