This happened to me a couple months ago. Sometimes, these people want something for free or a discount so they cause trouble on purpose.
I said I would reprint them but I needed the shirts back or she could buy them at a discount - she wanted them for free, she was NOT happy. I said no, I'll reuse them for samples so they won't go to waste. She refused to buy them so I just reprinted them. I still made money on the order. That B*tch took 2 months to pay me. I won't do work with her again. The mistake was not on my end but a friend had hooked me up with the order so I just rolled my eyes, smiled, and let it go.