Continuity mistake: When Dermot is drying off in the bathroom, he is holding the towel in his left hand and by his side, but in the shot of Debra he is holding the towel at his chest. Then cut back to Dermot and he is holding it by his side again.
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: When Debra and Dermot get to the big house and go into the room with the big bed, Debra walks into the room and her dress is hanging off one shoulder. Then when the back shot, her dress is off both shoulders.
Continuity mistake: Just prior to the wedding (near the end), when Debra and Dermot are outside the church standing beneath the arbor, she is holding her floral bouquet. When they enter the church, the bouquet is gone; Once she's walking down the aisle, the bouquet reappears.
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.
Factual error: When they are leaving the USA to fly to London and are checking in at the airport, a guy walks past in an Easyjet ground handler's uniform. Easyjet doesn't operate in the USA - clearly filmed in London Gatwick airport.
Continuity mistake: Both leave for London from NY in day time. Before they arrive and Kay wakes up still on the plane, Nick says good morning. It doesn't take that long to fly from NYC even in late afternoon and arrive in London the next morning (sun up) even with the time zone difference.