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.
Suggested correction: If you watch it again, you see after he rips the roof off and goes flying backward, the bottle actually flies back with him, and he actually grabs it. So he tucked it away or set it somewhere while off-screen to rip the roof off.