Five years passed, and the railing was never replaced. Not for any malicious reason, but the deck was never used because of it's safety issues, so replacing the railing was at the bottom of the laundry list of renovations to be done to the house.
My father was replacing the ceiling lights in their wardobe after we had repainted the ceiling, and after an exhausting day (and having finished replacing all the lights in the house!) he picked up the dust blanket he had down and took it out onto their deck to shake off. He leaned forward ever too far and in a momentary lapse of judgement leaned on the railing for support. The railing, however, had different plans. The entire railing gave way, and my father fell more than three metres onto the sloped lawn below.
Thankfully, my dad is an absolute lad and rolled when he hit the ground, meaning his right arm took the brunt of the impact. It did, however, completely destroy his wrist and, if it weren't for the muscle mass in the rest of his arm and chest, then far more damage would have been done. If you're worried, he's since made a pretty incredible recovery and has an almost full range of motion in his wrist.
The point of that story was not to find sympathy for my father, however: it's to demonstrate that even my father, one of the most intelligent people I know, can (and has) had mental lapses.
I feel I should point out that the deck my father fell through wasn't designed to be dangerous and simply became dangerous over time, however the deck was left unchanged because the assumption was that everyone wouldn't be stupid when they were near the railing.
A common excuse I see for lazy design is "well if you're stupid enough to break it, then you shouldn't be using it". Admittedly, this was an excuse that I subscribed to for a long time: I didn't want to spend the time to refine my UI/UX projects to accommodate everything that could happen, so if you didn't use it exactly as I would if I were to use it and then it broke, then "oh well, you're using it wrong".
Designing a car that doesn't alert it's occupants if someone isn't wearing a seatbelt under the assumption that every occupant will always remember to buckle up ignores the fact that people can be preoccupied or distracted, or be a child whose caretaker is preoccupied or distracted.
Designing a computer that instantly and permentantly deletes files under the assumption that a user will always self-check what they're deleting ignores the fact that misclicks happen, or people make mistakes.
Tall bookcases are designed to attach to the wall behind them for stability because people have children who like to climb, people may fall and grab the bookcase for support under the assumption it's stable, or any other circumstance wherein users may accidentally misuse something we design.
Designing something for "stupid people" isn't about designing for literally stupid people: it's about designing something so that regular people (who are often more intelligent than we give them credit for) can still use what we design, even if they're having an off day, they're pre-occupied, or don't understand what's going. We can't expect our users to know everything we do, all the time. They are human (as are we), and what we design needs to accommodate the fact that mental lapses happen and mistakes are made.