Similar story… only different, and with a good outcome:
Friends bought a nice AQHA mare, in foal, from a very big, well-known reining breeding farm, at auction. Received their mare (bought online auction) and liked her even better than they thought they would.
They even rode her a bit as she was only just in foal when they got her, and found the mare to be a much better horse than they thought they bought. She seemed almost ready to go in the show pen, great lead-changer, great stopper… just a very high quality mare.
They messed around with her a little that summer and fall, then waited for her to foal.
Which she did, a very nice foal that exceeded their wildest expectations.
Then they went to register the baby and were told by AQHA that the mare was not the mother of the baby which they had helped deliver! Clearly, the mare was the mother of the foal, but something was amiss…
After many conversations with both the seller and AQHA, they finally found out that the seller had DNA’d two identical plain bay mares the same day… and they got switched.
Fortunately, the seller honored their end of the deal and acknowledged that they had sold the wrong mare, unknowingly, to my friends. My friends had bid on, and expected, a nice mare in foal to a nice stallion… what they got was a great mare with a ton of training (although never shown) in foal to a much more expensive stallion. Which yielded them a very high-quality foal that they paid a modest price for as a 2 in 1 package.
Good for the seller to honor the deal which was their error-they ended up selling an elite mare in foal to an NRHA Futurity winning sire for a song to my friends. They had so many horses that they got the two mares mixed up initially, and they stayed mixed up for a long period of time, until my friends went to register one of the resulting foals.