I'd just about given up on the office coffee machine. I'd swipe my credit card, the machine would say it was processing, and then... nothing. No failure, no success. But about 1 out of 10 times, it worked.
I told the office manager, who said - "It works for me." I figured it was a network problem - the little cellphone antenna in a windowless break room just couldn't get a solid connection.
Then I was at a Memphis Redbirds game, and tried to swipe my card to buy some food. The teenage kid slinging fries had some expertise I lacked: "You have to tap it. If you swipe it won't work." That turned out to be true.
I hadn't even noticed that the credit card company had added a "tap" symbol on the card. I certainly had not thought that the coffee machine, whose directions said "Swipe or Tap Your Card" could be operating the same way.
But the next workday I tapped my card on the coffee machine. And it worked 100%. I would have been a loyal customer, maybe spending $150 a year on coffee.... but there was a small UX problem. The directions weren't clear. I was using it wrong, and had almost no way to find out.
The lesson is clear: watch people use your website, App or product. Watch enough people until you ruthlessly spot and remove these kinds of problems.