Continuity mistake: At Patty's place, she places her right arm next to her face holding a cracker. Shot changes to a side angle and her arm is lowered. Then a shot afterwards it's raised again.
Continuity mistake: When Fred Astaire leaves the cab he doesn't have anything in his hand with the box, but as he walks to the hotel he has a small bunch of flowers which he takes into the hotel with him. But they are never seen again. He does not give them to his date, nor does have a flower in his buttonhole.
Answer: Where the stairs were blown out they could have crossed over to the adjacent stairwell and back.