n!
$n!$ is the product of all positive integers less than or equal to n. $n$. In fact it should be defined in combinatorial terms.
Many assume the fact that parallel lines in Euclidean geometry do not cross is an axiom, while it can easily be proved in terms of vector space.
Many lecturers do not stress the difference between inner product and scalar product and most students think that these are different names for the same thing.
In complex numbers $i = \sqrt{-1}$. Obviously it is not correct as well.
