catastrophic intermed. shaft failure-why?

[wagonhead]

Hi! Newbie alert. Son's '90 Integra RS had a intermediate drive shaft failure. 2" axle stub left in the tranny--it came out with a magnet. The intermed. shaft carrier bearing mount broke, taking part of the block casting with it. Son says he did not abuse the car (???)
The odd thing is it happened after the shop put on a reman half shaft on that same side, approx. 80 miles previous to the failure. Doesn't appear that any of the broken stuff had to be messed with to just replace that half shaft on that side, so how or why might it have failed? Tranny is free, wheels are free, carrier bearing is free (the bearing a tad lumpy, but plenty free). Doesn't appear to have any impact marks like he ran over something.
Possible install of the wrong half shaft?? Critical part left off??
Son is not known to abuse the car other than to do stupid stuff like not checking the oil level, wacking curbs with nice rims, not steering around gaping potholes. Thanks for helping!!!

[wagonhead]

Went back to the site and gathered up all the parts I found strewn along the road after the car barfed its guts. Still missing some. After cleaning the parts with solvent, it is apparent that the mounting bosses of the engine block--where the bearing support mounts--was the failure area. One of the three bosses (at least, though I only found two) had heavy oxidation and discoloration along the lenth of the boss, along where the bolt would thread, indicating it failed long before the barfing time. It would have failed sooner or later. Moral?: don't abuse your cars as it gets expensive. If he decides to keep the car, it will at least need another short block.