Factual error: Basic physics - Hancock throws Michel from a dead stop to above cloud level in about eleven seconds. The clouds are bog standard cumulus which form at around 7,000 metres in temperate zones. This means that Michel accelerates to about 700 metres per second instantly, from a dead stop. Obviously he cannot accelerate during his ascent, so his starting speed has to be at least that. (In fact he would have to start his ascent much, much faster than 700 metres per second as he would be constantly decelerating due to gravity and air resistance, but it will do as a start point.) Michel accelerates from 0 to 2,520 kilometres per hour - twice the speed of sound - in zero seconds. He would be accelerating at around 5000 Gs, turning him into a very long streak of fine, pink mist.
Continuity mistake: When Hancock puts the woman on the hospital bed near the end of the movie when he walks out he walks into the metal bar holding the window in place and as a result it bends. In the next shot where you see the frame it doesn't even have a scratch let alone being bent.
Continuity mistake: In the scene when Mary visits Hancock for the first time at his house, she arrives from the air and crashes down on the ground, after a discussion inside they both walk outside together and Mary is shown getting into a car. In the next shot shown from above the house the car has disappeared and they both fly into the air.
Continuity mistake: (Spoiler Alert) the day after Hancock finds out that Mary also has powers, he stabs her with a large fork with two pointy ends. As expected, they do not puncture her skin. The pointy ends bend toward each other, making a triangle shape. But in the next shot, when Mary snatches it from his hand, the tips are no longer in a triangle shape, just bent the wrong way a little. Mary snatches it away by the handle, so we know that she could not have taken it by the points and bent it.
Continuity mistake: At the start of the movie, when Hancock rips the roof off the car and points to the baddie on his right, watch his right hand finger. First, he points to the man on his right and then to his left. In the next shot he is still pointing to the man on the right and then to his left.
Factual error: In the opening chase, Hancock arrives, promptly destroying highway signage that breaks up and collapses. At left on the screen, a large portion of this debris lands on the trunk of a police car, which then defies physics by having its front end (the heavier end) simply rise up off the ground. The car reaches a 45-degree angle, then the rear end leaves the ground as well as the nose lowers again, and the car is momentarily airborne. Nothing falls in front of the car to form a ramp that would send the car upward. The debris would have merely crushed the back of the car and perhaps brought it to an abrupt halt. No lever/fulcrum action would occur as the suspension would collapse first. Nothing like what we see is remotely possible.
Continuity mistake: During the battle between John and Mary, a huge storm brews up above and around them. Shots of Ray having a meeting show this storm outside the window. But just before the office is torn open, a momentary close-up of Ray in the office shows a normal, late-day sunny sky, then the wall is ripped open and the gray/blue storm is there again.
Continuity mistake: After Mary reveals herself to Hancock, the next time she and Ray are at the kitchen table, the towel Ray picks up off the table immediately changes completely from the one that Mary uses to try and open the jar. There is no other towel anywhere near them, and it's a split second, so it's definitely supposed to be the same one.
Continuity mistake: When Hancock is coming up on the SUV in the beginning, he has a bottle of whiskey in his left hand. When he lands and goes to rip off the roof of the SUV, you see his left hand empty (except for a handful of roof). After opening the vehicle, you see him fly backwards, and you see that he has the bottle in hand again.
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Continuity mistake: At the very end of the movie, just before the credits, there is a scene where a criminal (Mike Epps, uncredited) is running from the New York Police. In the first shot of him, he has a gun in his hand. Then he runs out in the street and stops as he is surrounded by the police. At that point, he does not have a gun. Then, as he grabs a hostage, he suddenly has a gun again right before Hancock lands to help apprehend him.