I found the point of failure. On the forum, most say the diff normally fails at full lock, in first or second gear, under moderate power: Basically when you’re slowly going round a corner or turning into a parking bay at a shopping mall. The dealer's chief mechanic says the diff only fails at that time, but it’s commonly as a result of previous damage probably when a front wheel gets airborne in a pothole and hits the ground again. The planetary gears take a massive beating and can partially split without breaking. The problem is that the surface is hardened too much, leaving it too brittle while the interior metal structure is still malleable enough to keep the gear together without splitting it.
The crack or break can sit there for years just waiting for that moment when you’re on full lock, the crack is in the right position and you release the clutch. It only fails with the crack in a specific position. The original crack can normally be recognized by a smoother face than that of a brittle fracture. Brittle fractures are caused when the gears finally gave in and shatter.
Three photos attached to illustrate it all…
Pic 1: The original crack. The whole planetary gear shattered because of this crack.
Pic 2: Brittle fracture will look like this. If the gearbox suddenly failed, then all the fractured teeth would have this rough appearance. The wave indicates the area of highest tensile force, i.e. where the fracture originated.
Pic 3: The mating surface of the original crack. Smooth means it reseated and the two surfaces polished each other smooth due to minute vibrations. The brittle bit was the only part holding the gear together.
My diff was also marked "GTA" in a black marker, but mine was an R-Spec unit meaning it added 50kW accross the rev range... See, it says GTA R on it!
Obviously, I'm kidding...