In a scene within Inception, Dominic Cobb is caught stealing documents from a safe by Saito and Mal. Saito's henchmen bring in Dom's accomplice, Arthur. Mal tests whether they are in a dream by shooting Arthur's foot and causing him pain. She also deduces by the luxurious decor that they are inside Arthur's dream. (I guess Arthur likes to dream about luxury.) Dom spares Arthur the pain by shooting him in the head.
Arthur wakes up immediately, but Dom and Saito continue inside the dream. Dom only wakes up after his sleeping body is dunked into a tub of water. That occurs a minute or more after Arthur wakes up. The movie says that time passes 20 times faster in the dream than in real life, so 20 minutes would have passed between when Arthur "died" and when the house was flooded.
Why would the dream continue if Arthur is already awake?