I worked for Dana 22 years, I have seen inside umpteen hundreds of carriers. (Mack Truck) There is an art to building them. There are very strict tolerances that if not adhered too well, does rear end whine ring a bell? Jigs are made to eliminate the constant micrometer measuring sometimes if the jig isn't positioned properly crap happens. Sometimes its the parts not coming from the vendor correct. I have seen this hundreds of times, Bull gears not right etc sent back to vendor. Sometimes they get through. Inspectors will randomly tear apart a carrier and go over it not every one is inspected.
The white paint you see on gears is there for a reason. The gear is marked to measure rotation to assure all gears are properly meshing.
I knew guys who could build these in their sleep. Excellent builders who were required to build x amount a day. We had cells and each builder was required to build a certain amount. They would work as fast as possible to they had more time at the end of the day for some slack time. This replaced the assembly line routine that gets really mundane and your mind wanders and more mistakes are in general, made. Still, the faster you work the more prone you are to making mistakes.
Then there are those who build with I don't really care attitude. The better the employer treats you the more you are inclined to put out better.
Despite it all crap happens I still don't know exactly what the cause of the rear end whines on these cars were a result of, I have heard theories but no definitive reason. What looks simple in the carrier is actually more complicated than you think.