The 304th prohibition is that we are forbidden from taking revenge upon each other.

This would be, for example if one person did something [against another]; and the other person doesn't stop pursuing him until he pays him back in the same way, or causes him the same pain which he caused. G‑d has prohibited this, in His statement,1 "Do not take revenge."

In the words of the Sifra, "How far does the prohibition against taking revenge go? If a person requests, 'Lend me your sickle,' and the other won't lend it; and tomorrow [the latter] requests, 'Lend me your ax,' and he answers, 'I won't lend to you just as you didn't lend me your sickle.' In this case the Torah says,2 'Do not take revenge.'"

From this example one can derive all other cases.