I got into a minor accident this week when a deer ran out. Most of the damage happened from losing control and going into a ditch.
The rear window shattered, as well as the wiper blade motor housing breaking. Front bumper, fenders, radiator support, etc. slightly messed up.
Engine died during the accident.

 

I pulled the right timing belt cover off and the gears literally fell out in pieces.
It was a 96 outback 2.5 with dohc, and plastic camshaft gears.

 

I know the work entailed to fix this, I'm just wondering how common this problem is.

I just didn't expect to see the both the lower and upper gears in pieces. Oddly the left side, especially the upper gear which appears as though the cam shaft gear was dropped at one point because its missing a piece, is still perfectly fine.

not common, but they break sometimes, most likely due to prior fatigue - being dropped, removed, pried, held in place to tighten the cam bolt, etc. 

a perfectly running, nominal pulley failure with no prior compromising incident is probably rare.