when you watched the pulley stop, almost, the center part was still turning, though you may not have noticed.

The center part is well connected to the crankshaft. No slip there.

The outer part is a fat ring with the grooves for the belts in it. that's what you saw stopping.

Between the two parts is a rubber ring, 2 or 3mm thick. When it fails, you get the results that you saw.

Now, visualize what can happen if that outer ring comes off the center part while the engine is turning 2000 rpm or so.

The harmonic balancer isn't the only part that will be shot. Maybe a radiator, alternator, hood, whatever that heavy ring hits.

So - put in the new one ASAP.

Good Luck,

Bob

:>)