I just purchased a very pretty 1884 Waltham movement that doesn't run and I'm playing around with it with the light-hearted goal of fixing it if possible (I'm not deluding myself as to prospects of success). I believe there are only two serious problems. The less serious of the two is that the hair spring is pretty rusty...although the balance still spins back and forth crudely. Doubtlessly, it would require work (replacement) to operate at a reasonable level. The more serious is confusing however. The third wheel (the one that connects the fourth "seconds hand" wheel and the second "hour-hand" wheel) is loose. It falls out of its socket off the side far enough to disengage the gear from the first wheel. It appears that it is also loose vertically, not just laterally, in that is slides up and down in its socket (thus slightly falling out of the jewel, but not so much that it slides around inside the watch).
What could possibly cause this? Obviously, I need to get in there with a magnifier or loop or something, but I was curious if anyone has any thoughts on this. It seems weird. I would expect a stem to either sit tight or be broken and disengage entirely, but not to sit loosely.