I've seen reversed polarity that changing the polarity resulted in switched neutrals. Then, you have to decide whether reverse polarity is worse than a switched neutral. It's the sort of thing that you never know about in advance. If the customer won't (or can't) pay for the repair, you can't do it for free. I like the drive thru oil change solution. Tell the customer and note it on their invoice. You have to trust that they will promptly have it fixed.
