Other mistake: In one of the first scenes where the girls are in the recording studio singing "Say You'll Be There" study carefully what each of them are wearing. I.E. Mel B is wearing a space suit. Later in the movie, when they're in Milan, they are all seen wearing the same things.
Deliberate mistake: Directly after Nicola has her baby, she sits up and holds a baby that is clean, umbilical cord-free, and has teeth. There is no possible way this could happen.