Continuity mistake: In the scene where Debra Messing and Dermot Mulroney first arrive in London at the first party, Debra's jacket belt is undone when they enter through the door, then done up tightly when they are shot from a different angle, and undone again as those move from the room.
Continuity mistake: During the "shower scene" when Dermot comes out naked and Debra looks away with her hand on her mouth, her hands keeps changing places. Facing her, she puts her hand on her mouth, but in a side shot, she has her hand over her eyes. Then when facing her, she has her hand over her mouth again.
Continuity mistake: The dance lessons take place the morning after stags and hens, making it Friday. But they're suppose to be two days before the wedding, so they should have been before stags and hens on Thursday. When Kat arrives at the cocktail party, from the airport, her mother gives them the itinerary. Tomorrow is young people in the park, followed by stags and hens, Friday is the picnic, then the rehearsal dinner. During the party, she's talking to her sister at the bar, after a stumble with the fiancee, Amy says they have dance lessons two days before the wedding.