The Z axis was recently behaving strangely.  I noticed the belts were flopping around so I retensioned them.  In a couple days, they were flopping again.  I took them off the machine to see what was going on and instead of finding that the steel cores had broken, I found this:

The polyurethane belt had slid/stretched over the steel core (of course it did!) inside the belt clamp.

I have redesigned the belt clamps so the ends of the belt will fold over and the teeth will interlock.  That should prevent this sort of problem from reoccurring.

I have used this folded self-locking type of clamp for years in the XY stage and in SoM, my other printer.