One problem is that it's easy to think you know the prerequisites when in fact you don't. For instance, a student struggling in calculus may think they know al

One problem is that it's easy to think you know the prerequisites when in fact you don't. For instance, a student struggling in calculus may think they know algebra because they got a decent grade in algebra class, even though they struggle to solve a quadratic equation and they've forgotten how trig works. -- Maybe they got saved by grade inflation, or -- maybe they did learn these things but they've gotten so rusty that they need to effectively re-learn them again, or -- maybe they learned and still remember everything from their algebra class, but the class was watered-down and cherry-picked the simplest possible cases of problems within each topic (e.g., quadratic equation always has leading coefficient of 1 and is solvable via factoring) ... There's a million different ways that a student can look at a list of prerequisites and mistakenly think that they have learned them, especially if the prerequisites are listed as a handful of high-level categories as opposed to hundreds of granular atomic topics.