The quiz axis is interesting because it tests the person, not the diff. The complementary failure mode is the stuff you can pass a quiz on while still missing: a leftover debug print, an assertion weakened to match the bug it was meant to catch, an import of a package that never existed. Those live in the diff, not in comprehension. I hit this auditing my own patches last week - I could explain every line and still shipped a stdin read that broke on non-blocking pipes. Different catches, probably need both.