Revealing mistake: As the gang is being chased through the "96th Street Station", you can see the true station name "Hoyt" (as in Hoyt-Schermerhorn Sts. Station) on the outer station wall. (00:42:50)
Revealing mistake: When Fox is thrown onto the railroad tracks in the subway, if you look closely at his legs you can tell that he stops in midair, meaning he obviously landed on something much higher than the tracks (boxes maybe).
Revealing mistake: When Ajax is handcuffed to the park bench and a cop punches him in the stomach with his nightstick, look at the police car behind the cop as he straightens up. The writing on the side of the car is backwards, indicating that the film was flipped to reverse left and right.
Revealing mistake: In the subway during the struggle between Fox and the cop, you can notice the stunt double in action instead of Fox.
Answer: There was a truce, but just in case you want different members in different parts of the crowd to see all angels and all exits in case of something. You plan if something goes wrong where everybody will be.