So aside from the details of this particular incident, why doesn't this happen more?
I want to be clear that I don't want this to happen, it's clearly a bad, bad thing when it does, but think of it this way: Each term (semester, quarter) millions of people enroll in classes. Tens of thousands of classes. Business 101, Advanced Operating System Design, Underwater Basket Weaving, whatever. Statistically it's very unlikely that any given class will fail (there's probably at least one person who's going to do the work well enough to pass) but over the whole set of classes, term after term, year after year, shouldn't we expect to see this happen at least once every so often?