backfeeding breaker box without shutting off main?

During the outage today we got all the standard warnings about not backfeeding because it can kill linemen. I can see that happening if your house has the broken wire, but if it is a large area, it seems to me (speaking from a position of almost complete ignorance) that if you failed to turn the main breaker off, you would effectively be trying to feed the entire region, and your generator would stall. So, if you are running, it is an indication that you did in fact shut off the main breaker.
Does this make sense, or am I over looking something?

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