Chapter 1, Mishna 5
בָּאוּ אֲחֵרִים וְהִזִּימוּם, בָּאוּ אֲחֵרִים וְהִזִּימוּם, אֲפִלּוּ מֵאָה, כֻּלָּם יֵהָרֵגוּ. רַבִּי יְהוּדָה אוֹמֵר, אִסְטָסִית הִיא זוֹ, וְאֵינָהּ נֶהֱרֶגֶת אֶלָּא כַת הָרִאשׁוֹנָה בִלְבָד:
Let's say other witnesses come and make the original witnesses zo'me'min (plotters), and then other witness come and they make them zo'me'min, even up to 100 pairs of witnesses- all will be killed.
Rebbe Yehuda says it's like a pot of ink that makes everyone dirty, but cannot do this forever. Only the first pair of witnesses are killed.
CONFUSED???? You should be! There are two ways of understanding this Mishna.