[Shalom Israel]

It is quite possible in math class for more than one student to make the same error (eg, getting the sign wrong when rearranging an expression). If this is repeated over and over, then...

That's true, but I'm not thinking of the "typo" sort of errors. Usually, errors reveal a misunderstanding of the concept itself, and misunderstandings tend to be like fingerprints, especially in higher-level courses.

[Virginia-American]

[errors in math class]

I remember once in an algebra class:

question: list all the groups of order 91

next session: Well, unfortunately, a lot of you thought 91 is a prime number...