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.
Revealing mistake: When the older Saint boy is pointing his gun at Frank's chest, you can see the squib under his shirt, you don't actually see him get shot until the flashback later in the movie, but you can see that the blood explodes from the same spot.
Revealing mistake: When the car Howard Saint is tied to explodes, watch Howard's body - it's clear that he's been replaced with a hilariously obvious dummy. It's especially comical if you look at his legs, because it appears that the dummy doesn't even have legs inside of its pants given how they "wobble."
Revealing mistake: When Frank kills Howard Saint at the end of the film, pay close attention when the cars start to explode. About a second-and-a-half after the first explosion, the camera cuts to a high-angle shot looking down at the cars exploding. Pay attention and you can see a lot of errors. For example, there are two cars (one red and one blue) on the right side of the screen that are teeny-tiny models and aren't properly blended into the background with blurry edges around them. Additionally, the three cars on the bottom left of the screen also have noticeable black lines around them. (Likely from a poor compositing job.) It appears the shot was created with different miniatures layered over a background plate, and the various miniatures weren't properly comped into the shot.
Revealing mistake: When the Punisher's wife is driving madly away from the Saints with her kid, their Jimmy flips over and rolls, and we can see the gas tank has been removed. Also notice the vehicle is fully caged and we can see the pipe cannon go off underneath the vehicle as it flips the car, also visible as the car lays on its side.
Revealing mistake: When Saint shoots one of his henchmen for not taking care of his son, when Saint throws his gun on the body, the henchman jerks.
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).
Answer: The song is "La donna é mobile" from the opera Rigoletto.