I had a student do something similar once upon a time. Near as we
could figure, he got the shank of the tool caught between two out of
the three jaws of the jacobs chuck. That’ll hold it enough to spin
it, and even do some very light work, (so long as you don’t mind (or
notice) the eccentricity (or thumping.)
But as soon as you put any real pressure on it, it’ll pop loose.
Depending on what happens next, it can get fairly dramatically angled
before it slides itself back between two of the jaw teeth. They then
proceed to grab the shank at whatever extreme angle it managed to get
itself to, and whirl it around. Usually slamming it into something
(like your finger) which proceeds to bend the shank.
This isn’t as hard to do as you’d think, and depending on how you
did it, the working head of the mizzy might have been more-or-less
centered. So the thumping wouldn’t have been very great. Not any
worse than mizzies sometimes have anyway if you mount a new wheel on
the mandrel. I’ve done it once or twice when I was in a hurry, and
changing over from a large shank to a small one, or if I’d really
opened up the jaws for some reason. (you mention that you’re using a
thick mandrel…) Now, I’m paranoid enough that I usually eyeball the
chuck once, just out of habit, to prevent such entertainment.
Of course, if you’re using a collet handpiece, this can’t happen, so
just ignore the foregoing.