My 2008 Forester and I took a hard right turn a few days ago when the outside temperature was about minus forever up here in Vermont. The left rear axle popped out of the differential. 

 

We put it on a lift and shoved it back in, and its been fine since.

 

Could extreme cold have locked the c-clip in a compressed position somehow, or...?

 

I'm at a loss to explain system-failure-to-system-heal-thyself situations.

No way, that's definitely not normal.  It was no colder than zillions of other Subaru's endure all over the country. 

 

I'd check for warn bushings, look for rear movement of the hub assembly and rear diff hangers.  I guess i'd get it off the ground and pry hard on various places looking for play and listening for noises. 

 

History - have you owned this since new and it's got 100 miles on it?  Or what?

How many miles have *YOU* put on the current rear tires and are they wearing normally?

Has the vehicle ever been wrecked?
Any major rear repairs?

I've owned it since new. That rear wheel bearing has been replaced twice since new though...hmmmm...

 

Never been wrecked, 265,000 miles original everything except put an Imprezza's 2.5 in it at 235,000 when the original went bad due to valve guide migration.

 

I'll check the diff mounting hardware next time I can put it on a lift - at 265K I'm going to replace the diff, driveshaft, and 5 speed tranny (2nd and occasionally 3rd gear synchs going) in one shot as the engine only has 145K and the body is remarkably rust free.

Good,

 

Simplest explanation would be the axle was accidentally pulled out during the wheel bearing job.  How long ago was the last one?

 

Did you top off the gear oil after that incident or did any gear oil puke out during that incident?

 

Those diffs never fail, I'd just leave that.  
Synchro's yeah that's unfortunate. 

could some kind of snow/ice build-up contribute to this happening?

 

but, as said above, the initial problem was likely left-over from the bearing work.

When you go back in there buy new clips for the axles. They sometimes get slopply or chewed up and don't hold well. We have had problems on lifted Subaru's with axles coming loose. Especially aftermarket axles. 

 

GD