Ok...this is going to be hard to expalin and it's also possible that i've totally lost my mind, but i'm pretty sure i see what i think i see.
In "The Lip Reader", when Kramer is telling Jerry that he had a deaf cousin, there's a 2 second period where it absolutely looks like they punched in Kramer's voice over something else that he said. Right when he's talking to Jerry while using sign language, he says "I had a deaf cousin who lived with us for about a year", if you look closely while he's saying those words, it doesn't match his lips and mouth. It's as if he said something else and they changed what he originally said by punching in his voice with another line....is it me???