Revealing mistake: After The Russian smashes The Punisher's refrigerator door against his head a couple times, he shoves The Punisher to the floor and pushes the refrigerator door shut, but the refrigerator door suddenly and completely stops moving midway before closing, revealing the door to have a stopper built in so as to not actually hit the actor in the head with it (acknowledged by the director in the film's commentary track).
Continuity mistake: When Saints's men are killing Castle's family, there is a shot of Frank and his dad shooting at one of the guys behind the barbecue stand. In one shot Frank and his dad shoot the salad bowl and most of the salad comes out, then in the following shot the salad has magically reappeared. (00:22:30)
Continuity mistake: After Frank Castle climbs out of his flipped Pontiac GTO, Harry Heck walks up and points his shotgun at Frank. Frank points a closed switchblade knife at Harry and pushes the button, sending the knife blade flying into Harry's neck. When it switches back to Frank, there is still a blade on the knife, in the open position. After a cut to Harry, it switches back to Frank and the knife blade is gone.
Continuity mistake: When Frank's family is being attacked, Frank and his father are inside the house fighting with some thug hiding behind the barbecue stand. At one point a shotgun shot is visible in the black metal lid on the left of the stand. In the next shot, the lid is intact, and then when the gas bottles blow up, it's shot again.
Continuity mistake: When Saint's men murder Castle's family, the man who tried to escape on the motorcycle is shot and falls off next to the motorcycle. The motorcycle slides and comes to rest with the back of the motorcycle facing the large deck and ocean. However, when Castle chases his wife and son, he picks up the motorcycle, and it is turned around with the front facing the ocean.
Visible crew/equipment: In the shot where the 4x4 flips, you can see a stunt driver and a dummy inside the vehicle.
Continuity mistake: When Frank sits up after being dragged out of the ambulance by his friends, he takes off his jacket with his hands/arms behind his back, but in the following shot his hands/arms have switched and are now at the front. (00:05:35)
Continuity mistake: When Frank crashes his Pontiac after speeding away from Harry Heck, Heck pulls up and walks up to Frank who is climbing out of his car. As he walks up, a piece of car is on the right side of Heck, but after Frank kills Heck, it cuts to an overhead shot and the piece of car is now on his left. (01:04:00)
Continuity mistake: When Castle and Joan are talking and Joan is touching Castle's face, there is a shot where her hand is no longer touching his face. In the following shot her hand is back on his face. (01:11:00)
Continuity mistake: When Frank is pulling down the gravestone with a chain, in the shot from the front, William's is to the left about a foot away and Maria's is to the right, the same distance. In the shot from behind the stone, there is another stone you can just about see on the right of the screen and it can't be William's because this stone, or something, is directly behind Frank's. (00:44:25)
Continuity mistake: At the end, when Howard Saint is being dragged behind the car into the lot full of burning cars, he is catching on fire in a close up. Then in a far shot and Saint is no longer on fire, but in the next close up he is on fire again.
Visible crew/equipment: When Howard Saint's men begin to chase after Frank's wife in their red 4x4, just after a couple of guys jump in the back, the camera pans forward to get a view of the driver. As the camera pans forward a boom mic with a large pole is visible in the reflection on the car. (00:24:20)
Continuity mistake: When the Punisher is eating in the diner, Harry Heck walks in and when he stops at his table the red chair is tucked into the table, but in the next shot when he sits, the chair is facing out. (01:00:00)
Revealing mistake: As the truck that Castle's wife is driving flips, you can see a burst of smoke or compressed air under the passenger side. Once the truck has stopped rolling, you can see a piece of U-channel bolted across the underside with a cylinder on the passenger side, which was used to help the truck roll over.
Continuity mistake: When Frank walks up to Saint's son, Frank waives a white flag, but between shots his position changes from being really close to Bobby, to being at least a foot away and his body side on. (00:02:25)
Continuity mistake: When Frank crashes his car and Harry Heck pulls up in his, note the distance from Heck's car to Frank's car, perhaps only a foot, yet when it cuts to an overhead shot, Heck's car is now quite some distance away from Frank's without anyone moving it. (01:03:50 - 01:04:25)
Continuity mistake: Whilst being chased by Harry Heck, Castle drives through a railroad gate. As he does so a piece of wood seems to get jammed or stuck on his right headlight. In the very next shot, the piece of wood is nowhere in sight, not even on the road.
Continuity mistake: When Frank returns to his apartment after shooting the two guys in the lobby of the Saint building, the two tenants, the big guy and the small guy with the piercings fall out of the door and onto the floor. When they land the small guy has his left arm outstretched above his head, but in the following shot it is no longer outstretched. (00:50:25)
Continuity mistake: When Saint's men knock down Frank's wife and son, Frank arrives and starts shooting at the 4x4. The wife and kid have their heads and bodies some distance from each other, but a few shots later they are a lot closer, and their heads are almost head to head. (00:28:00)
Continuity mistake: When Howard's men have killed all of Frank's family and are chasing after his wife, they catch up with her and fire upon her 4x4. In the first shots her left brake light is out, but in the following shot its back on, and also if you zoom in you can see the stunt driver with two strong belts going down each shoulder. (00:25:40)
Answer: The song is "La donna é mobile" from the opera Rigoletto.