I have stumbled across such phrase in the TV show House:
The police investigated what they saw, not what they didn't saw.
My question is why is the second form of the verb "see" is used in the second part of the sentence, but not in the first form as it should be used in past simple after "did"? It is supposed to be didn't see, isn't it? Is it some kind of emphasis? And in which cases can it be used?