I'm after some advice on possible causes pls - just fitted a replacement gbox from a scrappers on my sons car. Drove it forwards okay but then on selecting reverse and dropping the clutch it went bang and spewed all it's gearbox oil.
Looks like it's thrown a bolt out through the diff housing, which is a new one on me, could that be the case ? (no pun intended)
I'm 100% certain i didn't drop anything into it whilst fitting, and i bought it as a sealed unit.

 

It's a common failure. Something breaks or gets dropped in the gearbox and finds it's aay between the diff wheel and the casing where there's almost no clearance.
Bang
Hole in casing
Had it many times on my classic Mini