We had a piece of software that would present a dialog for the user to select a file, then parse and send the contents to an embedded device. This software was an internal tool and not very reliable, so it would crash if you selected a file with invalid contents.
Two engineers, Dan and Brian, came to me with a tricky problem. Every time Dan would open a specific file everything would work fine. Every time Brian selected the same file the program would crash. I was obviously skeptical and went to watch. We tried it 10-20 times and sure enough, it always failed for Brian and worked for Dan. I watched them perform the exact same steps.
Eventually I gave it a try myself and realized what was going on. Dan would click "open", select the file, then click OK. Brian would click "open" then double click the file.