So, sometimes you
really don't anticipate the kinds of failures you can have. A really good one to remember is the "Camera Shy" Pi 2.
For those unfamiliar with this issue, or who forgot: The power regulator IC on the Pi 2 was a flip-silicon slab that was otherwise uncoated or encased. When engineers were taking pictures of it, they likely were using their cell phones and otherwise loose lighting under fluorescent or sunlight. When high intensity Xenon bulbs were used for flash photography, however, intense IR light would hit the silicon, excite something, and cause the power supply to go wobbly kneed, drop out like a Silicon Valley startup founder, and you'd have a bad day.
This failure was only figured out when someone happened to compare two flashing lights (one Xenon and one a bike light) that they figured out what was going on.
Engineers miss things all the time, even those who are supposedly really good. They are good, but humans err, because to err is human.
Prime example: I diagnosed an arcade cabinet that only had issues on sustained playtime during the summer, but never during the winter. It was fine on the off day that there was a lot of cloud cover, and only seemed to get worse after a shade awning was taken down: before the removal, it would intermittently sputter back to life during the later afternoon. Nobody knew what was going on.
I spent an afternoon watching it fail. At one point, I eventually went and grabbed an IR thermometer and pointed it at the cabinet. It was registering well into 100F. It was also outright refusing to work.
I rolled down the window blinds, pointed a fan at the poor machine, and waited for a while. Miraculously, it woke up after a while and started working.
I traced it to the three canned voltage regulators. The voltage regulators for the system were strapped to the metal chassis of the CRT, which contained the system's power supply. The machine happened to be right next to a window, and the cabinet had been painted black by the previous owner. As the daytime sun would wander over, the black paint would soak up the UV and absolutely cook the regulators. Once the sun had passed behind the shade, it was quickly cool enough inside the chassis to bring the regulators back into spec, which caused the machine to start working again. When the shade was removed, it only went back up once the sun was no longer cooking it like a roast chicken.
I believe my recommendation was "Replace the side paneling with woodgrain and move it out of the sun."
The engineer that designed that arcade machine probably never thought it'd be in a hot, New Mexico university faculty lounge next to a window that faced the sun. I remember going back about a year later and sure enough, it had moved about three feet and was "rarely, if ever broken" now.
Engineers miss things all the time.