In the mid-70's, I repaired aircraft electronics for "general aviation," i.e., non-commercial aircraft. To understand this story, I need to give some background:
Switches which should never be accidentally flipped were covered with "switch guards," which were hinged plastic covers held down with soft copper wire which could be easily broken when needed.
Safety wire was stainless-steel, high-strength wire used to lock things in place.
I climbed into the cockpit of an airplane for a flight test (I was in the co-pilot's seat) and noticed a pair of wire cutters tucked into a pocket on the console.
"Why do you have wire cutters in the cockpit?"
"I carry them ever since a mechanic safety-wired my fuel-dump switch covers, and I needed to dump fuel. I ruined my nail clippers."
Long ago, developed a small software for a restaurant for their Billing and Inventory. The users, mostly servers, were comfortable using the numeric Menu Item codes while entering an order, so the system allowed them to enter the Item codes.
Got a frantic phone call one Sunday around lunch time "Your software is not accepting any input". To check that it is not a computer or keyboard problem, asked them to open some other application (like Notepad) and enter something. They reported no problems with entering data in Notepad and insisted I be there ASAP to solve the problem as it was their peak business hour.
Made the 45 minute drive to visit and found out - they have turned off the Num Lock and were trying to enter the numeric Item codes via the Num pad. No, they didn't try the non-numpad number keys even once. Turned the Num Lock on and everything was working fine.