The worst thing I've seen a developer do would be becoming a university teacher while not being able to understand simple OOP or good programming practices.

I can't think of a most harmful one, just mediocrity cultivating unknowingly mediocre devs.

That's the case of every teacher I've had in school. There is one special case of a fucking stupid fuck that I will never forget. I'm from Mexico and my first language was PHP, so this fucker asked us for a completely well built system that did whatever we wanted but had to be "big". So I built this basic "store" with crud and inventory and all that stuff. The day of the presentation an error related to converting a date to a string popped up, the huge fucker spent entire weeks talking about his trips to NY and how much money he made and how good his english was. Well, the error was clear "can't convert date to string" and I told him, oh dude, that's cos of the server... the server is not working very well and he was like... "uhm, alright, go on" OMG fucking asshole, since that day I realized how much I hated that school and most of my "teachers"

@bennythecat96 I'm also from Mexico and I totally get it.