About 8 years ago I was working on a mobile game where you could purchase specialized dragons and eggs. Some of these could be pretty expensive, but since they were high end items we wrote special GPU shader code for them so they had cool special effects on them. We tested these as well as we could -- we had a room with maybe 100 or so mobile devices -- but of course we couldn't test on everything.
One day an irate older lady came to our office, and our receptionist for some reason let her in (probably thinking old lady = harmless?). Keep in mind our office was unlisted because we didn't want fans dropping by. She had driven all the way up from Arizona to Colorado (although I don't think it was the only reason she drove up), and she accused us of ripping her off, because she had bought one of these fancy dragons and instead of getting what she saw in the promo materials, its wings were black! I didn't hear or see this directly, instead it was the main topic on our Slack chat with everyone being cautioned to Play It Cool.
I didn't think much of it until I realized it was _my_ code that had caused this entire issue in the first place!
Luckily we had a really good customer service guy that defused the entire situation, but that's the first and hopefully only time I've been tracked down in person by a customer for a bug.