Comment by rchaud

As a senior dev or manager, you're responsible for the people you've hired. Their mistakes become your mistakes. If they make the same kind of mistake repeatedly, and aren't able to take responsibility, you will have to clean up after them. They're not able to fulfill their job description and must be let go. That's why the probationary period exists.

Realistically, the issues occurring here are intern-level mistakes where you can take the time to train them, because expectations are low and they're usually not working on production-level software. In a FT position the stakes are higher so things like this get evaluated during the interview. If this were a real person, they wouldn't have gotten an offer at Microsoft.