To me, the definition of junior is someone who needs guidance. If you don't have a good process of defining tasks before they're worked on and real code review before things get merged, you are not set up to work with juniors, and hiring them is doing both yourself and them a huge disservice.
Junior job titles are nearly as much of a mess as interviewing them.
Particularly in recent hot topics like AI the reality is you want both experienced and less-experienced devs. Most AI teams I see actually lack AI experience and keep falling for the same issues (like quickly getting to 80% working and thinking just a bit more data to 99.9%)
In this case, nobody really kept an eye on how things were being built, and the person was actually remarkably productive. I really don't have a bad word to say about them: they were given vague tasks and came back, for most of them, with working implementations. We're only a handful of people and everybody was busy elsewhere, but what little guidance and review we could offer was always accepted with much gratitude. They lasted about 18 months before moving to somewhere that offered more support, and I can't blame them at all.
What happened is predictable: things that ought to be in one place end up distributed in little pieces, and existing features got invasively modified to add new features even when that wasn't necessary. Poor separation of concerns, design patterns applied wrong, all of the stuff that over time adds up to a codebase that's hard to work with.
I think a lot of the refactoring we do would make a good book, actually, if any of us were the type to write about programming.
...doesn't quite square with:
> I'm still cleaning up after the unsupervised junior two years later.
I can appreciate that misfactored code now helps "flatten the curve" and skirt under whatever deadlines you were respecting——even at the cost of a long tail of reactors——but its a shame that junior dev isn't around to learn from their mistakes, and also that they weren't paired with a more senior dev who could've coached them through those reactors at the time, enriching both the codebase and the human capital.