Continuity mistake: When Steve Martin is on the spinning carnival ride, the positions on either side of him are empty, and then in the next shot both spots are occupied.
Continuity mistake: During the gas station scene, Steve Martin is flashing a wad of dollar bills in front of the Latinos in the lowrider. From one camera shot, there is a $50 bill on top, but on the next shot there is a $1 bill on top.
Continuity mistake: When Steve Martin runs from his apartment and throws popcorn into the detective's face, the detective spits out popcorn even though no popcorn went into his mouth. (01:00:50)
Continuity mistake: When Steve Martin is working at the gas station, the Latinos come in with the stolen credit card and he puts the gas nozzle in the gas tank under the license plate. It is still in there when he hooks up the chain from the car to the church. But when he walks back from the church, the nozzle is mysteriously gone from the car, and has returned to the pump. (00:22:10)
Continuity mistake: When the phone book arrives, Navin (Steve Martin) points to his address on the right side page. When the sniper (M. Emmet Walsh) picks a name out of the phone book, his finger falls on the left side page.
Continuity mistake: When Navin is at the gas station and is being shot at by the sniper, take a look at the arrangement of the oil cans on the rack beside him. One of the cans in the middle of the rack changes color from pink to purple, and some labels turn, between shots.
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).