The foot-hole in the windshield suddenly, inexplicably, changes sizes from small and jagged, to larger and more rounded.
The day Saul and Dale wake up in the woods is Daylight Savings Day, which is always on a Sunday. However, Dale visited his girlfriend's high school the day before which should have been Saturday when it should have been closed.
When Saul is hitch-hiking using his thumb as a penis, his watch is clearly black. In later scenes his watch is silver.
When Dale first visits Saul to buy pot, the tray on Saul's table has two empty silver canisters. When Saul returns with Dale's Pineapple Express pot, they reach for the cross joint and one of the canisters is filled with pot.
During the car chase the hub cap falls off, then keeps reappearing and disappearing.
General Bratt's uniform is wrong for 1937. It is from the 1970s. He should be wearing what was called "Pink and Tans". Additionally he seems to be wearing a marksmanship badge. Officers don't wear these. He is also wearing General Staff branch insignia. Generals don't wear branch insignia. Finally his shoulder epaulet should tucked under his lapel.
Several solders within the military bunker are armed and not wearing cover hat indoors. Military rules clearly state that all armed personnel shall wear a cover when under arms.
Ted Jones has no connection to Dale until Saul tells Red Dale's full name. The degrees of separation only lead him as far as Saul. Therefore, Dale and Saul could have just hidden out at Dale's apartment from the very beginning. However, Dale can't be certain that the murderers didn't see him or his license plate, so it makes perfect sense that he'd want to stay away from his apartment.
When Red is bleeding in the bathroom after being shot, it may appear that Dale calls Red by the name of Dale. He actually says "I think 'Yes! I'm going to help you, Dale.' That's the answer I'm looking for!" He was trying to motivate Red - he didn't call Red by the wrong name.
When Angie is talking to Dale on the phone in the hotel, it is visible that the phone is not plugged into anything when Angie is moving the base around and swinging the wire.
When Red starts beating Saul with a dust buster it bends every time Saul is hit, revealing it to be made of foam.
In the beginning of the film when the private is smoking a joint, the camera cuts to outside of the box he is sitting in and you can clearly see the smoke from a smoke-machine blow towards the private.
In some shots during the cruiser chase, there is a wiped off area on the Slushee-spattered windshield next to Saul's foot to let the stunt driver see out.
When Dale and Saul are fighting Red in the house. At the end when Dale grabs Red and yells, "This ends now!" Saul mouths the same line.
In the Trailer "On the hood" the computer in the police car is pulling up data on Dale Denton. After it shows "Wanted for Felony hit and run" it then says "arrest on site". The correct usage is "arrest on SIGHT"
After Dale witnesses the murder and goes to Saul's apartment building in a panic, the sound of the rapid buzzing as he pushes the call button doesn't match the buzzing heard in Saul's apartment.
While Dale and Saul are talking in front of a huge mirror in the holding room of the farm, two crew members' reflections are clearly seen.
When Mr. Anderson is shooting at Saul & Dale, there is a small flash in the lower cupboard door as the charge is set off. A bullet/buck shot would not create this flash.
Dale asks Saul for a quarter of an ounce of Pineapple Express, but the bag Saul gives Dale clearly contains less than an eighth of an ounce.
After Red kills Matheson with his Daewoo Lanos, he says the famous line "You just got killed by a Daewoo Lanos, motherfucker!" When he says it, he pronounces Daewoo incorrectly; the W is silent and it's pronounced dayoo.