As a young CS student who had self-taught before taking a formal curriculum, this was one of the things I found the most disgusting about the state of high-schools CS ed.
When the exercises and grading code have bugs, that's a major problem. When the instructor can't recognize those bugs when you point them out, that's even worse.
I moved on to tutoring and teaching CS later in life, (on the side, as a mentor with a FIRST Robotics program,) and encountering bugs in the curriculum like these were some of the things that set my students back the most.