I am thinking it might be items that only cost them a few dollars to make...going by the description.

so if, say, it was a major t-shirt producer...I would let them wear their error for four tops, but would become a loyal customer.

some stores also have flash sales on a few items to get noticed. I have bought items at 5$ that were back up to 40$ the next day, from Target. Less extreme but still quite a discount. If an error it would be difficult to know what to offer...as the OP was not going to buy them at the new price.