Designing to prevent common mistakes – even if they're stupid mistakes – is a good idea, but you can't idiot-proof absolutely everything. At some point personal responsibility starts. I think this is somewhere before "I'll use a frayed extension cord from the 70s" – even a child can understand that's not a good idea.

Where I roughly draw the line is where the user was stupid. People should know not to be dangerous with extension cords; but there's no way a user should be required to know that the fifth outlet box on the left side actually has no box and that if you were to pull out something plugged into it the whole outlet would come out of the wall and short to a water line.

And perhaps this kind of thing should be more explicit in the code - things that are for convenience and things that are actual life/death safety issues.

And as we see, the latter often involves things that only become apparent when disaster strikes.