Here's a weak spot that has nothing to do with initial breakage but causes a diff to go boom while you are driving home from a trip (I know this one 1st hand), the drain plug on the axle housing!!! Knock it loose on a rock and it falls out halfway home from your trip, gee I was wondering where all the oil that was collecting on my rear window was coming from! 
Good point! Been there; suffered that as well. It's certainly worth doing a tightness check before getting on the road, or just tacking them shut instead of blowing up a rear diff.
