I read with interest recently that a substantial number of Tesla S owners are getting their drive units (electric motor + gearbox) replaced by Tesla due to premature failure. The early symptom of the failure is an audible gear noise which Tesla techs call "milling". Edmunds and Motor Trend both had their cars fail in this way.
According to a Tesla Motors Club poll, here, 77 members have had their drive units replaced once, and an astounding 15 have had them replaced more than once.
How bad is this?
It is hard to estimate because we don't know how many cars form the total sample for the poll. Tesla Motors Club indicates 17,995 registered members, while total Model S sales in the U.S. are around 28,000 units (39,163 globally).
So let's play with the numbers a little.
Assume all members of the forum own Tesla Model S:
92 failures / 17,994 Tesla Motor Club members = 5.1 failures/1000
Assume all failures were reported in poll (seems unlikely):
92 / 28,000 U.S. sales = 3.3 failures/1000
Assume forum represents 5% of all Tesla owners (I saw this mentioned on InsideEVs.com):
92 failures * 20 / 39,163 = 47 failures /1000
In the auto business, a major component failure rate north of 5/1000 units is considered a major quality issue, and anything approaching 47 is a complete disaster.
If, for example, the 2013 Toyota Camry had a similar rate of transmission failures, somewhere between 5-47/1000, that would translate into repairs of between 2040 and 19,000 transmissions! At the top end of that scale, likely a recall.
In other words, the "milling" issue is a major quality issue for Tesla. I am sure they have their engineers and suppliers pounding away at it, coming up with design fixes to reduce the occurrence of this.