Instability of long cylinders

Instability of long cylinders
From time to time on these maillists the subject has come up that spheres and tori might be better shapes for habitats than long cylinders because of the latter's tendency to eventually tumble end-for-end. While on the newsgroup rec.arts.sf.science somebody was talking about the ultimate size for space habitats given current materials, and I was mentioning the dimensions of Island 3. This prompted James A. Donald to say:
"Island 3 is not practical. Being longer than it is
wide, will be unstable. Will flip over, killing
everyone inside."
To which I responded:
"Eh, you're referring to a real-life aspect of physics, but needlessly
hyperbolizing it. It's more like if civilization fell in an O'Neill
cylinder, and the stabilization systems failed, and if you went away and
came back after a few decades, it would be tumbling end-for-end rather than
on its long axis.

"Frankly, I consider the mirror design of Island 3 to come closer to making
it impractical than the simple fact that it's longer than it is wide.

"But the point relevant to Knobby's point was that cylinders 4 miles in
diameter are possible with present materials."
He came back to say that "It would tumbling be end for end in about ten minutes or half an hour" and further went on to say that an above-ground swimming pool breaking and dumping its water would be sufficient to destabilize the cylinder.
So what say you guys? A problem if the automatic stabilization system remains off-line for weeks or months, or a catastrophic flip within the hour, killing millions?
But ultimately, it occurs that my very first response should have been that an O'Neill Cylinder(s) habitat would actually have 0 net angular momentum because it's actually two counter-rotating cylinders. In The High Frontier, O'Neill only mentioned this as a solution to the problem of turning the axis of the habitat so as to track the sun, but could it be that this design solution was also proposed for the second reason of the instability of long cylinders?

Regards,
Mike Combs