Godwin's Law

As the number of people engaged in a debate or discussion increases, the probability of a reference to Hitler or the Nazis approaches one.

There are two important corrolaries:

The first person in a debate to compare his opponent's position to Hitler/Nazis loses.

Once a discussion reaches comparisons to Hitler/Nazis, it is no longer useful.

Nota: substituir "Hitler/Nazis" por "Chamberlain/Muniche" também acho que funciona...