Continuity mistake: When Stuart is leaving the building, he enters a previous generation Mercedes Benz S Class. The next couple of shots where he is being followed by Bryan shows the newly redesigned S Class.
Continuity mistake: When Bryan calls to Kim to tell her mother is dead, at the beginning she has no socks but few seconds later when the talk is finished she's wearing one black sock.
Continuity mistake: On the highway, the police car he's in is struck twice on the back end. Impact actually pops the trunk. When the car drives off, there's no damage to the car.
Continuity mistake: When the police are watching the funeral, Kim hugs the 2nd man with her right arm over his shoulder and around his neck, and her left arm behind his back. When it cuts to the surveillance camera, Kim's right arm is lowered and holding his arm.
Continuity mistake: When Liam jumps in the water from under the truck, he goes completely underwater. When he is walking through the wash, his sweatshirt is half dry. (01:25:45 - 01:27:06)
Continuity mistake: When the Russian was coming down the steps in the apartment shootout he has on black bottoms, but when he is shot they turn white.
Continuity mistake: When Oleg is dying on the stairs in his apartment, we can see his back is resting on the fourth step. A couple of seconds later his back has somehow relocated to the third step after death.
Answer: Some water might have been used, but actually waterboarding an actor would be a safety hazard. Most likely some special effects are involved.
raywest ★