Actually, posted some info on that from your last Tech Post
This is on Lucky (The 2005) so I don't feel too bad about the age/fixing of the issue, but still think there is a weak point in the design with the plastic to metal housing they use and plastic wheel. (wish the plastic was metal)
After replacing one side not long after your original post, the other window has since had the same issue happen and I will need to be doing the same fix yet myself. I don't have a warranty, and it is a pain to get the whole window assembly out and a new one back in..
not to mention the $250ish hit
As for a trend... After 4 Mini's I have only had a water pump on an 09 and the 2 window assemblies on the 05 to fix so far.
(besides regular maintenance)
Good luck! (Get it into for that warranty!)
-- Here is what I originally wrote
After just replacing the Quarter Window assembly on one a couple weeks ago, I would indeed say there is a weak point in the cabling wrapping around the plastic wind gear/wheel...
My issue was the plastic that holds the cable actually broke (plastic snapped in two) which prevented the entire window from moving, but in your case it sounds like with some movement it may be an issue with the cabling kinking or getting out of alignment.
Glad it is under warranty as you can't buy individual pieces, if something breaks with the guide or cabling it is about $270 to buy the entire assembly and metal frame. (plus labor if the dealer does the work)
Good luck on the fix... I'm sure Tempe will get it fixed up (I think they quote 3hrs labor on this after talking to the parts guys)