Just a little quibble.

Technically the proper use of the term "cog" is a tooth or projection on a wheel or bar. I know that we call the rear sprockets "cogs" very commonly in the bicycle world but really it's a misuse of the term. So stevage is the more correct by referring to the.... er... sprocket as a "sprocket".

I've seen the chain derail and jam up the RD on many bikes that don't actually have anything at all wrong with them. It seems to occur moreso when the cross chaining is such that by backing up the chain is fed off the sprocket it's riding on and over to the next one or two. How far over it goes depends on the amount of the cross chaining angle at the moment. The same thing would happen if you back pedal the cranks at that point instead of pushing the bike backwards.