R&D mode doesn't enter into it, really, from what I have read (only had mine this past weekend). Someone speculated it's the turning on/off that can do it. Or rather, if the machine is prone to breaking, it may do so when you're flashing and rebooting multiple times to do the R&D mode thing.

I put mine in R&D mode, then fixed the gainroot script so I could gain root without R&D mode, then reflashed back to non-R&D mode and have had no worries thus far (knock on wood.)

All electronics break eventually, just the luck of the draw. I wonder what the percentage is for the 770? One does need to keep in mind that when it breaks, people sound off about it, but when it just works people don't go posting "Mine just works!".