Near the end, when Velma shows up to Roxie's not so great audition...Roxie is putting on her fur coat, buttons it, turns around and its open! Then, when she leaves the room and comes back in...its buttoned up again!
This is not a correct slip-up, though it's an understandable mistake to make. Roxie only closes the very bottom button on her coat. When she's walking out, you can't see the closed button, but if you look you can see that the lapels are facing inward to show that it is closed lower down.