I'm not sure if this was already known to many people, but I never experienced it before. Note that this "hint" has no productive use, unless you personally benefit from trying out GUI variations.

From time to time, I'll hold down Shift while I perform various GUI tasks to see if I can slow them down -- yes, OS X still impresses me enough to want to view its animations more thoroughly. However, I accidentally held down Control one day while minimizing a window and noticed a slight difference in the speed in which it flowed to the Dock. It wasn't as slow as holding down Shift, but it wasn't as fast as normal.

Apparently, Apple included this function as an in-between speed. However, when both Shift and Control are both held down, minimization is almost excruciatingly slow. I figure this could be used for, if nothing else, checking to see how smoothly your GPU and Quartz 2D are working. I tend to give my system a pop quiz every once in a while when the processor is busy, by slow-mo'ing a minimize operation just to see if the system is on its toes. ;)

[robg adds: I searched the database, and I don't think we've explicitly mentioned the "Control-only" speed for animations before, though we have discussed Shift-Control for very very slow motion animations.]