Window regulators on late model BMW's have been consistently unreliable. The really strange thing seems to be that the one that almost always fails on the X3 is the passenger side. In the last 3 months, there have been 8 or 9 cases on this list only, and as best as I can recall all except one have been the front passenger side regulator, The one other was a rear passenger side unit.
I'm wondering if it's a case of passengers rapidly reversing the window (going from "up" to "down") without stopping for a second, which is causing the regulator cables to jump off the idler wheels and jam? As we all know, BMW DRIVERS would never do such a thing!
Just a thought.