Trivia: Not a mistake, but a little extra. When we first see Doug the Head in the bar drinking and getting his lunch, the man on the right, reading the newspaper is director Guy Ritchie.
Trivia: Due to the modest budget, the production team could not afford enough extras for the boxing sequences. Therefore, the crew would shift the same people from one shot to the next to make it seem it was like a crowded hall.
Trivia: Brad Pitt almost turned down the role of Mickey O'Neil in this movie as director Guy Ritchie informed him that he would be playing a boxer, and feared being typecast as he had already played the same role in Fight Club (1999). Pitt took up the role anyway, as he desperately wanted to work with Ritchie.
Answer: It saves time, and it's a little twisted. Would you chase after a guy who you just saw cut off a dead guy's hand to get a briefcase, then just decided to take the hand with him?
Captain Defenestrator