When I was still a noob programmer, I was working on a website for a big client. We had a demo coming up in big city. So we drove there several hours and went to their office. All the management board and shareholders and what not were there.

So we started the demo. Everything had worked perfect the night before. But on that day, we were right away greeted with some stupid PHP error right there on the first page. Had to fix it quickly so we could continue with the demo, so I logged into their production server with SSH and started fixing the code with vim. I was connected to the projector, so my horrid noob code with cringy joke comments was there for everyone in the room to see.

Eventually got it working, but I saw several people in the room facepalming hard. Can't ever forget the day. :D