The "Tennis Racket" theorem-- it's all there! Take a look (especially at the
video clip made by a Russian cosmonaut in 1985-- the effect is named after him).
The video looks kind of like my 3-axis rotation example-- but even more weird!
Yet it's real. If one of my own animations had turned out that way, I would have
said it was *completely* unrealistic.

https://en.wikipedia.org/wiki/Tennis_racket_theorem

I also came across this ACM paper-- well, the link anyway. (Not free,
unfortunately; I haven't read it.)

ACM (Association for Computing Machinery)
"Free-fall motion synthesis" 2011
https://dl.acm.org/citation.cfm?id=2077386

My own 'practical' notion about all of this stuff-- so far-- is basically this:
It seems that rotation in two axes actually dampens out or eliminates rotation
around the third axis, for physical reasons (angular momentum, etc etc)...
perhaps depending on the degree of the initial rotation(s). And it may not be
proper or realistic to stuff just ANY values into rotate <x,y,z> (which is only
a 'stand-in' for the real physical processes anyway.) In the real world, the
values appear to be interdependent.

It's all quite complex, and I'm STILL learning...

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