Once, in a previous job, I had a customer who had a legitimate complaint. The company I worked for had screwed up, this customer suffered for it and had every reason to be livid. He made it clear that he was dissatisfied and that he expected us to fix the issue, but he was unfailingly gracious and courteous about it. Because of that, I, and several other employees bent over backward to solve his issue. It was a nice object lesson for me about how to treat the employees of a business when something goes wrong that is not directly their fault.
He was so delighted with our resolution of the issue that he insisted on giving me a bottle of wine as a thank you. I told him that I couldn’t accept tips of any kind and that the manager would only pour it down the drain, but he insisted. I told the manager, in keeping with the rules. The manager told me later that he poured the wine down the drain, but there were no witnesses, and he smiled unnervingly when he said it. That, too, was a lesson for me.