Other mistake: Mac is leaving a message for Christine. There is a close up of his cell phone. The keypad on the phone has the numbers in the wrong order. On every phone in history the first row of numbers is "123". On Mac's phone it is "789", like a computer keyboard.
Continuity mistake: The dead body of a girl is in the autopsy room. There is a closeup of her face on a TV monitor. Her hair is wet and tucked under her head and the v-shaped stitches on her torso are reddish in color. Next shot her hair is spread out and the stitches on her torso are blueish.
Chosen answer: Probably to broaden the scope of the show's plot and give the audience a chance to see the characters in a different setting, People act differently at home from the way they do at their workplace. By the ninth season, the characters would have become overly familiar and predictable. It gives the writers a chance to do something different with them.
raywest ★