Division by Zero

Mistake
Let $x$ and $y$ be numbers such that $y = 0$.

The quantity $\dfrac x y$ is undefined.

It is a common mistake to forget this fact when evaluating formulas.

Also see

 * L'Hôpital's Rule: while $\dfrac 0 0$ is undefined, the limit of a rational function whose denominator approaches $0$ is not necessarily undefined.