Revealing mistake: Bernadette Peters has a fight with biker chick. Biker chick flinches before Peter's hand gets anywhere near her face. (00:48:00)
Revealing mistake: When Bernadette Peters throws a knife at Steve Martin, it never leaves her hand. (01:18:05)
Revealing mistake: When the sniper is shooting the cans, the close-up of the first can shows the metal peeling out as though the bullet had come from inside instead of outside.
Answer: Navin self-applies the "Jerk" epithet early in the film, using it not in the sense of someone obnoxious or mean (as it is used in most cases), but in the sense of a stupid person. He's saying he blames himself and his lack of intelligence (though, as you say, it is usually simple naïveté) for ending up where he is (broke, homeless).