On Lee auto-indexing turrets, there's a nylon or plastic wear ring that serves as the sacrificial link in between the lowering/raising ram and the index rod that rotates the turret head. It exists so that you don't break or wear something expensive, and it's an easily-replaced 25-cent part.

If you're careful when you swap turrets, this part lasts for thousands of rounds. If you're not, well...1500-2000, depending on the number of swaps.

Anyways, when it wears, it gets rounded-off where it contacts the index rod, so when you lower the ram (which should advance the turret), you get either a partial advance, or no advance at all. The wear ring just slides down and up the twist in the index rod that, if the ring contacted it right, would have spun the turret.

It'd be real hard to miss this when it happens. The lever pull feels wrong, even if you didn't watch the press. And most of the time, you get a partial advance where the turret stops halfway between the spring-loaded detent points. But--and this is key--people are idiots.

So that, and some kind of failure with the powder measure, are what I worry about. And really, mostly the powder measure, because I watch the press pretty closely. Close enough that I can actually see the powder charge drop from the charge hole on the disk through the powder measure, so I don't even worry about that too much.