Has anyone failed a student teacher? If so, why?

I had a student teacher this past school year and reluctantly passed them. They had a great presence in the classroom and connected really well with the students but lacked in content knowledge. They did well teaching the freshman but not so much the seniors I had (their lack of content knowledge was very apparent with them. I’m talking multiple mistakes a lesson). They seemed unprepared and would take lessons from my examples. I passed them but am partially regretting it. What are some reasons others have failed their student teachers?