Thank you for posting this. In fact, thank you to 1726 for posting your results of various tests and efforts this season, I'm sure you have inspired several teams to try new things as a result.
However unless I misunderstand your description of the problem, I get the sense that this may be a real fluke, rather than an inherent flaw in the gearbox... I'm not saying it didn't happen... obviously it did, however teams have been using similar methods of attaching the pinion and key for years without any problems. So, just to make sure I understand...
Referring to the tough box user manual
http://lib.store.yahoo.net/lib/yhst-...erGuide1-1.pdf you had part number 18 (the 2mm x 2mm x 14mm key) slip out past item 20 (the 5/16" clip)?
When I rough out an estimate of the spacing between the "teeth" of the clip, it seems they should be about 1.6mm apart where they contact the CIM shaft.
Theory says this shouldn't happen... practice and observation says it did. (At least if I understand this correctly.) Perhaps it was something specific to your team's modification of the gearbox, or how the retaining clip was installed?
Jason