Good point, but not the greatest example. 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...
IMO, better examples are 1) mapmakers who include false features in order to trap plagiarists, and 2) finding the same sentence structure, misspelled words, etc in 2 students' work, or in their work and wikipedia.
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.