It does seem unlikely; if your main breaker is closed, your genny
should stall immediately when it tries to power the neighborhood.

I think unless you have some sort of huge generator the load presented by
the outside world will look to the generator as pretty much a short to
ground. It would really have to be a situation where the power outage
resulted in you and your immediate neighbors being islanded. Where I live
there are 10 houses on the same pole transformer and even if that were
isolated I doubt my 4.4 kW machine could pull that load. Maybe if it were
the middle of the night and everyone's fridge just happened to finish
cycling before the power went out and and and... I think the risk is
nearly zip. But for safety to your machine more so than risk to any
lineman it might not be a bad idea to attach a reminder sign to your double
male cable or adaptor or whatever you do this with: TURN OFF MAIN BREAKER
BEFORE USING