Well, now I have experience. The tape takeup mechanism has a leader with a pin bound to it, running across the width of the tape leader. That pin is supposed to be either in its little parking/docking clip or attached to the clip at the head of an inserted cartridge's tape.

In one of my cases, it was floating loose. I don't know how that happened.

In the other, it was still attached to the docking clip when it should have been attached to the head of the tape. It appears that the tape's clip had gotten bent and the tension on the pin was too great for the docking clip to be able to extract it.

I managed to spend my entire day at work today dealing with this one issue.