Theoretically it should not happen if you have an automatic transfer switch. If you have a manual transfer by plugging in thee coach cord to the gen. outlet you can't possibly do that. You would be putting power into the grid and most likely not on the same phase. If power is out and you are using a generator at home you could very well electrocute a lineman working on what he thinks is a dead wire. A good system would be to unplug the generator if that would happen to you in a CG.