> barring rare emergencies
It doesn't have to be a rare emergency. Things happen, loud student parties happen, random bad nights happen.
In the case of calculus as an example, there should be nothing from the class you didn't have time to practice sufficiently. A full academic year is more than enough time to learn integrals, derivatives, sequences, and series with the small collection of tricks for solving difficult problems (telescoping series, trig sub, etc.) There's a couple dozen rules you learn that are straightforward to apply, and for the times they aren't obvious, there's like a dozen total tricks (trig sub, etc.).
I took these classes. I remember how most people just phoned in the entire thing and complained at the exam review sessions how hard everything was.
It's the simplest of memorization.