Ok, I know about not pedaling backwards in the big-big combo (and not using the combo in general) due to extreme cross-chaining.
I got my SRAM Force build all done and it shifts flawlessly. I'm super happy with it and feel that is is pretty dialed in. This evening (with the bike on the stand) I had the chain on the big chainring up front and the small cog in the rear. This is the least extreme chain angle (I think). I spun the wheel backwards and not long after, the chain fell off in front starting at the bottom of the crank. It's totally repeatable.
Why? Should I accept this as normal? My old 6600 stuff never did this.
All of the parts are new. Chainrings don't look bent. Rear D hangar is straight. Rear D looks straight...I hope it is. It's brand new.