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.
Visible crew/equipment: Carnival biker chick goes through a flaming hoop and falls down after going though it. Just before she goes, you can see a rope tied to her to pull her off the bike after she passes through the flames. (00:33:30)
Revealing mistake: When Bernadette Peters throws a knife at Steve Martin, it never leaves her hand. (01:18:05)
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)
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)
Visible crew/equipment: In the gas station scene, when Navin returns with the tires for the Latinos, a crew member becomes visible in the office window where he just came from. The crew member jumps into view and seemingly realizes it and hides.
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.
Visible crew/equipment: Mike boom shadow can be seen on the lower left portion of the screen when Steve Martin is talking to his "father" outside the barn. (00:08:50)
Audio problem: When Steve Martin is chasing the boy riding the train, while he is shouting, his lips are way out of sync with his voice. (00:39:50)
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.
Audio problem: During the duet "Tonight You Belong to Me" on the beach, Steve Martin is strumming the ukulele out of time with the audio, and he's not making the correct chord changes on the fingerboard. You can also see his mouth bungle the lyrics, although the overdubbed audio is fine.
Factual error: Marie plays a horn solo on the beach. At times continuous sound is coming out of the horn while she is inhaling through her mouth. Horns only produce sound when the player exhales.
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).