I have just managed to find another way to crash the game using princes.

First, some well known facts:
1) When a unit is in charge of a stack, even if by itself, its commander appears on the commanders list in the Military menu. Clicking on his name centers the screen on him. If he is in the retraining queue then the province is highlighted instead.
2) When you click a name on a royalty list, this centers you on the prince in the same way.
3) Princes are often commanders.
4) You can retrain princes in provinces that can retrain their bodyguard unit. Stacking more than one prince obviously results in a training queue.

So my Hospitaller princes have been drained in a ferocious battle against Turks. I send them back to Rhodes for refills. I select them while they are outside the castle, and drag both and one other drained knight group of 1 into the queue. There they are.

At the same time, since I've just conquered Asia Minor and my GM's about to die, I search around for my commanders and princes looking at their stats. A whole generation of princes is about to be disenfranchised afterall and they're going to have the stats to take up +star provinces like Trebizond. Meanwhile I'm looking to see if any of my other commanders have loyalty issues that make them especially suited to Lesser Armenia. Suddenly, I realize I have princes in the queue and also princes inside the castle. At some point, one of these princes appeared on one or the other menu and got centered while I had the castle and the retraining queue selected. This did not remove them from the training queue but did produce a doppleganger inside the castle because the thing really wanted to center on something. When I then removed the princes from the retraining queue, this spawned another quantum instance of them outside the castle.

Predictably, when i tried merging them into a stack, this produced a crash. :p