Brilliant juniors are most common source of this in my 20 years of software dev experience. The usual 'they were busy with how it could be done and didn't check

Brilliant juniors are most common source of this in my 20 years of software dev experience. The usual 'they were busy with how it could be done and didn't check if it actually should be done'. One of the worst things for creating long-term technical debt is to let such bright folks run free. KISS principle above all, when you look at products from really long term perspective and treat people as replaceable (which they always are and all will eventually leave), absolutely nothing trumps that. Unless of course somebody plays politics and create some cathedral for them to maintain, and only them. Which is behavior that should be recognized early and 'punished' accordingly.