What happens when a instructor really goofs up a course?

Given how many college courses are taught every academic semester in the world (or even just in the U.S.), it simply must be the case that some portion of the instructors badly goof up their courses, to the point where it is not really able to be recovered from--in essence, the course is ruined. This could be things like:

• Provably unfair or wildly too strict (or too lenient?) grading.


• Losing exam scores (this has to be particularly common) for some or all of the students/exams.


• Missing a large number of class sessions, or somehow leaving mid-course.


• Etc.

I taught for a while and luckily never had such a catastrophe, nor did I know of any from my colleagues. But what typically happens when this sort of thing does occur?