Single window mode (purple title bar button) in OS X Previews
From the party-like-it's-2000 department:
A couple of the Mac OS X developer previews had a weird purple button at the top right corner of window title bars, where the magical lozenge can be found more recently. This button appeared in a few developer previews, but was removed before the first really crappy public release.
This button was actually a toggle between normal layered windowing and a nasty “single window mode” which allowed only one window to show at a time. Single window mode simply minimized the existing (single) window whenever an action opened up a new window.
For more on this behavior and screen captures of what the button looked like in disabled and enabled states, see John Siracusa’s review of OS X developer preview 3 (start reading around ¶5).
The magical lozenge which occupies the same spot in more recent Mac OSes has two purposes. First, you click it to show or hide the toolbar. The second purpose — and the reason it’s a magical lozenge — you ⌘-click it to cycle between the various icon/text and toolbar size configurations. Like all forms of magic, this one has its downside as well, in that the odd ugly duck application may manage to do something just totally insane when you click the magical lozenge, at least until they fix the thing.

