It only goes in one way
One day I went out on an in home TV service call. I took a less experienced technician with me.
When we looked at the set, the problem was obvious, I'd seen it many times before.
This was in the days of modular TV's, I told him to go out to the truck and get the appropriate module.
While he was out, I noticed some radio equipment in the next room. At the time, I was heavily into Ham Radio, and so it turned out, was our customer. We hit it off and had an animated discussion on the subject.
My assistant came in with the module.
He went to install it, and asked which direction it went in. Not wanting to interrupt our conversation, I told him that it only fit in one way. True, at least by design.
He put it in, and I had him power up the set to check the results.
Smoke started pouring out the back of the set. Not good.
I checked out the problem. It seems that if one was willing to use enough force to break off the corner of the module, it would indeed fit in backwards.
Well, my new friend was no longer very friendly, he was now an official irate customer. He was yelling and getting insulting.
Unfortunately we didn't have the needed parts on the truck, so I had to go back to the shop.
While driving back, he called the shop and insisted that someone else be sent out. They refused and said that it was me or nothing.
To this day I don't know if that was a vote of confidence, or a punishment for not paying enough attention to what the new guy was doing. Maybe both.
It all worked out in the end, but I spent a lot more time fixing his set than I liked to do in the home, it was now much more than just a module replacement.