Would that only happen if one outlet were wired backwards (hot to neutral and vice-versa)?

If both outlets are wired correctly, as I’m envisioning it, the extension cord is parallel to the existing branch.

Same if both outlets are wired (the same) incorrectly.

But if one outlet is wired correctly and one has its poles swapped, then you are shorting hot directly to neutral…

But also being a 3-prong outlet…assuming everything is wired correctly…the short would either be redirected to ground and either trip the breaker or trigger a GFCI…no?

IANAElectrician. Just a high dude thinking about circuits.