There was a problem with the polarity in the extension cord.
Was this a "homemade" extension cord, one with the ends added later? Normally there wouldn't be a polarity problem with a manufactured extension cord with a ground pin. Usually the polarity problem is with the wall receptacle being wired incorrectly.
Glad it's solved, and, yes, we do learn from these experiences.
Normally there wouldn't be a polarity problem with a manufactured extension cord with a ground pin.
It's not even a likely problem with manufactured two-prong (no ground wire or pin) cords, because for many years they have been polarized (the neutral blade is wider, and connected to the neutral on the other end).