How bad is this going to be??

[lbhsbz]

So the question is how the hell does something like that happen? Someone not torquing the crank bolt to spec?

That's exactly how it happens. The key simply locates the pulley in the appropriate position on the crank...the (correctly torqued) bolt clamp the TB pulley between the balancer and the shoulder on the crankshaft to retain it.

 

[absalom]

In this thread, OP declared "luckiest man" of 2018 ...