We are going to talk about saying the Amida incorrectly (either on purpose or accidentally. 

We first learn in our Mishna:

When the Chazan is repeating the Amida, if he says:

"Hashem, you have mercy even on the birds in the nest"

or

"For the good we remember your name"

or

"Modim, Modim (We Thank, We Thank).

We make him be quiet (and get a new Chazzan).

WHY???  What's so bad about this?