When Rachel slaps Mike, Mike already has slap marks on his face before the 'first' slap occurs. They can be seen in the close-up as he deliberates telling Rachel his secret.
When Mike and Rachel are in the file room, Rachel turns around to allow Mike to unzip and pull the dress down to her waist. In a couple of the subsequent shots it alternates from being pulled down to her waist to being back on to back to the waist again.
Although the episode takes place in Manhattan, it was filmed in Toronto. A red and white Toronto streetcar is seen behind Dana when she talks to Harvey outside.