My recent version: I was playing a pinball game in an arcade. One particular ramp shot was registering earlier in the day and then stopped working.
Eventually I realized that the sensor is an optical beam, and the receiver happened to be in direct sunlight coming in through a window! So it was continuously receiving infrared and would never report the beam being blocked by a pinball. Sure enough, it started working again once the sun angle changed by a few more degrees.
I have an optical smoke detector that will give (very loud) false alarms if a sun beam can bounce off a windowsill onto it. It works great if the curtain is closed. Debugging that took a few early sunrises.
Heh, but not exactly. If I blocked the sun, the receiver still would have been picking up the real beam. I would have had to block the sun _and_ make the shot with a pinball at the same instant... which is just playing the game normally with extra steps.
Eventually I realized that the sensor is an optical beam, and the receiver happened to be in direct sunlight coming in through a window! So it was continuously receiving infrared and would never report the beam being blocked by a pinball. Sure enough, it started working again once the sun angle changed by a few more degrees.