I had a similar problem once. Some users were reporting that buttons were missing when the software was fullscreen. I spent ages chasing down Java painting glitches trying to figure out why they weren't showing. But while it always worked on my system, some customers still had problems.
Eventually it hit me that my code assumed a 4:3 screen, and there are a lot of screens on the market nowadays with a different ratio. Duh!
My boss has a story similar to the one told. A very angry customer could not get the program to work. No amount of consultation over the phone could find the problem. Eventually he drove out there - six hours - to see what was going on.
The laserdisc was upside down.