When Navin first receives the glasses from Stan Fox, the part of the glasses that hook around the ears have almost no hook to them. After Navin installs the nose bridge handle (the opti-grab), the glasses have normal ear hooks.
When Navin is being pursued by the sniper at the gas station, the sniper fills up the car's rear end with bullet holes. Later, after Navin escapes in the car without rear tires, there are no bullet holes visible on the car's rear end.
When Navin is talking to the credit card thieves, you see some small bills ($1 bills) on the bottom of the pile closest to the driver and larger bills closest to Navin, but when the angle switches to over Navin's you see the bills closest to Navin are now large bills ($20s, $10s).
When Navin realizes the credit card is stolen, he walks backward past the car. However the gas nozzle and hose hasn't been removed from the car but he passes the car without coming into contact with the hose.
When the new phone books arrive, Navin finds his name on a right-hand page in the phone book. However, when the madman randomly chooses a name to be his victim, he points to Navin's name on a left-hand page.
Navin opens his door and throws popcorn in the face of the Private Investigator, right after the popcorn bounces off the face the PI spits a mouthful out. The PI never opened his mouth prior to the throw.
When the madman is shooting at Navin at the service station, when the cans are shot at, the holes are formed from inside the can not from the shots fired at the cans. You can clearly see the aluminum fold outward from the holes in the cans.
When the closing credits are rolling, the girl in the white tube top is seen dancing behind Navin's mother. She then dances in front of the mother and down the stairs to the left. In the next cut she is seen dancing behind Navin's mom again. Navin is then seen moving behind his mother on the porch and the girl is gone.