When Frank Castle's wife and child are in the Blazer trying to
escape, it flips over, while flipping the hand on the steering wheel is wearing a black bracelet, when his wife crawls out of the car she is wearing no bracelets. The "black bracelets" are safety devices for the stunt driver, similar to what Sprint Car drivers wear to prevent their arms exiting the profile of the vehicle during "accidents" or stunts
During Castle's final raid at the Saint's estate, he fires several shots into a henchman with his pistol and follows up with a "tactical" reload (replacing a semi-full magazine with a completely full one.) Castle then racks the slide to chamber a round. There would be no reason to do this as there is already a round in the chamber. If the pistol was in fact empty, the slide would've been automatically locked to the rear.
The second time Frank places a fire hydrant by Mrs. Saint's car, he is briefly wearing the short sleeved Skull t-shirt (from the previous time he did it), but in the wider shot he is wearing the long sleeved trench coat.
When the guitar player comes into the diner, he sits down and, while smoking his cigarette, opens his guitar case. As he does so, his cigarette repeatedly jumps back and forth from his mouth to his hand between shots.
When Frank is being chased by the Heck (after he gets over the bridge), it shows the metal shields are pulled down on both doors, but in a shot when it shows him turning, you can see one of the shields is no longer there, but in the next shot, it is back.
During the shootout at the family reunion, one of Saint's men is shooting from behind a barbecue grill at Frank and Frank's father. As Frank and his father are shooting at Saint's man, a bowl of salad on the counter is sent flying in the air as Saint's man dives over the barbecue grill. The next shot shows Saint's man shooting at Frank with the salad bowl full of salad on the counter. Then we see Frank shooting at the barbecue grill with the salad bowl gone and salad all over the counter.
After the massacre at the Castle family reunion, Livia and Howard Saint go to the Saints and Sinners club. They give Micky Duka the newspapers where it says (in the lower left corner) that Frank's 10-year-old son has been killed. But in the scene when Frank pulls his tombstone out of the ground, you can see the tombstone of his son William Castle where it says 1995-2003. That means he was only eight years old when he was killed, not 10 as it was stated in the newspapers.
When Frank Castle is escaping from Harry Heck he drives his GTO off of the bridge. In the close-ups of Castle just before he hits the Chevy Lumina, you see him pull the steering wheel hard to the left, but in the scene the GTO goes right. He turns in the opposite direction the car actually goes.
In the initial FBI bust sequence, the drug dealer is holding a gun at his side and pacing around. When he finally acts, he reaches into his jacket to pull out the gun that he was already holding out in the shots before.
When Frank is being chased by Heck, and Heck starts shooting at Frank's car, all the bullet holes (caused by Heck) disappear as Frank is driving over the bridge. But, they reappear after the car flips.
As Castle is having dinner with Joan, Dave and Bumpo, right before his line, "Thanks for dinner," his left hand is on the table and his right hand under the table on his leg. In the very next shot, both his hands are on the table.
After Frank has dinner with Joan, there is shot of Frank walking toward the window. The shot changes to Joan's perspective, and we see Frank looking up towards the curtains. In the next shot (directly after) there is a close up of Frank's face and he is no longer looking up, he is looking out the window.
In the "Russian Fight scene" at the apartment of Frank and Joan the Russian throws Frank through the wall and follows by breaking the door to Joan's apartment. When they leave the house after Joan sewed Franks stabbing, You can see Joan's apartment door in the background - unbroken.
When the truck thats chasing Mariah and Will hits the hills in the sand and gets a little airborne, the men in the back are seen flipping around like dolls. When the camera comes closer one man is on top of the cab and then in the next shot he is flipping backwards again.
When Castle shoots one of Saint's henchman using a bow & arrow, you can see that the henchman is shot in his back whilst looking towards, whom we see after the cross blur - Castle. This is impossible as in order for the arrow to enter the henchman's back he would need to face away from Castle.
When Harry Heck comes in at the diner, all the chairs are facing their respective tables. He bends over to put down his guitar and in the next shot, he is sitting down on a chair facing towards Castle, away from its table.
Frank Castle and Jimmy are talking sitting on the park bench. The sound of a motorcycle is heard and continues uninterrupted through to the next scene where Frank Castle and Jimmy have left the bench area to Jimmy's car.
The same scene of the Saint limo driving along the street passed the street vendors is repeated twice. First is traveling to the casino and second is when traveling to the bridge over the train tracks. Note the woman holding up the t-shirt while facing toward the camera.
When Castle first steals Saint's money, the millions of dollars are all in bundles of hundred dollar bills and are placed in a large cart. However, in the next scene when Castle dumps the money onto the streets from the high-rise window, all of the bundles are separated and the individual bills float down to the street. There is no way that Castle had time to unwrap all of the bills and then throw them out the window.
When Howard Saint asks Micky the date on the ticket in the scene where he finds out about his wife's fake affair Micky says it was "September 5th, last Thursday." In the year the movie is set, 2003 (as shown as the year of death on Frank's tombstone) September 5th fell on a Friday.
After Frank is shot on the pier he later returns to his Father's house and retrieves his Father's guns, since it was a crime scene the police would have removed all the weapons from the scene and not left them for anyone to steal.
The truck that is used to kill Castle's family (in Puerto Rico) has a front license plate. License plates in Puerto Rico are only displayed in the back of the car, as you are only given one plate for the car.
In the beach shootout scene when Frank's family is killed, Frank and his father are trying to kill a man who has taken cover behind the outdoor BBQ. Frank kills the man by shooting the propane tank with his shotgun causing it to explode. However this would not actually happen, shooting a propane tank with a gun, even a shotgun would not make it explode. Now the shotgun might have enough power (depending on what kind of loads Frank was using) to penetrate the propane tank, but it would not cause it to explode. In order for the propane to explode it would need an ignition source. Most people think that shooting a metal can with a gun will make a spark, but this is not true. Almost all bullets have a copper jacket and a lead tip, and neither lead nor copper make sparks when they hit metal. While Frank was using a shotgun, which has BB's instead of bullets, almost all shotgun pellets are made from either lead or copper, so they would not create a spark.
During the massacre at the Castle estate in Puerto Rico, when Maria attempts to escape with Will, she tells him to "run to the Jeep." It's a Chevy Blazer, but "a Jeep" is commonly used as a shorthand for an off-road car, not necessarily of a Jeep make.
The location of the blood squib under Castle's shirt on the pier lines up with the hole in his shirt after he is rescued. The resulting scar from his gunshot is higher and more to the center of his chest rather than the location of the squib/hole in his shirt.