I had my diff built by a local manufacturer (very reputable) last November. The invoice listed all they did which included filled it with gear oil. I know I should have checked this prior to installing it but I didn't. While I was going through the checks prior to the first go-karting session, I checked the fluid and it was bone dry. Pumped in the required 2 L of oil and replaced the fill plug. Went through the go-kart runs and noticed that the diff leaks. I contacted the manufacturer and after back and forth emails, he had this to say.
"Daryl,
My sincere apologies.
Unfortunately since November none of my staff can not remember the actual particulars of that build and pickup.
But seek comfort in knowing this will NOT happen again to anyone. I have instituted several measures to insure every sealed unit that goes out of here is filled with the correct fluid.
This is the first unit in 20 years that a unit has not been filled with fluid. That was my son!
Good thing you did your due diligence in checking the level prior to installation.
Stop by the shop sometime when I am here and we will remove and replace the diff cover and change the oil and extend the warranty one year from that date.
Many thanks for letting us know."

and when I told them that the bolts were not fully tightened AND there was no sealant on the cover, I got this back.
"Daryl,
We all have been scratching our heads how this happened.
Was there any sealant (silicone) between the cover and the diff case?
It looks to us like we may have not sealed the cover and left the bolts loose because the C clips axles were coming out again to do some disc brake work?
Which is often the case with our custom builds, when a client is suppling his own brakes.
What do you think?"

After fully tightening the bolts it still leaked, just not as bad. Their response to this was:
"Daryl,
Bring her in, we’ll fix it.
XXX 988 6815."

Needless to say, I won't be returning it to them and I will drain it and replace the cover with a new gasket and sealant.
This is just a reminder that no matter how highly rated some company's might be - check and verify.