I have fortunately never witnessed it myself- but some fellow sparkys have seen first-hand just what happens when the PoCo and a generator get in a duel.
Best description: Shrapnel and molten metal flying aeverywhere, as the genny goes "ka-boom!" Or the faulty transfer switch. Or, whatever component is the "weak link."
We're not talking about a little smoke coming from the windings; nor are we talking bout a stalled motor. We are talking about the equivalent of a very big line-to-line fault. Something has to give- and it won't be on the PoCo side!