First of all, I would like to confess that I, more through luck and reading instructions, rather than having any special talents, have hooked up a few gennys, battery banks, and so on- without any transfer / backfeed problems. May this streak continue....

BUT...I know of others who have been present when the generator power was slightly out of phase, and ran into the PoCo power.
NO, there was nothing as gentle as a breaker tripping. Rather, there was a big "BANG", a blinding flash, and molten metal flying everywhere. Often, this arc-blast took place at the panel, or the transfer switch, or some other place remote from the genny.
One of these accidents -of which I have a first-hand account- resulted in a fatality (the guy who threw the switch).