Continuity mistake: When Sky is getting his hat and cigar as "props" for his stag party, as he leaves the room wearing the hat and holding the cigar. In the second shot as he walks past Sophie, there is a long loose cord hanging down from the brim of the hat - which wasn't there in the first shot.
Revealing mistake: After Sophie faints at her bachelorette party, during the next scene it cuts back and forth between shots taking place in two rooms - Donna walking into Tanya and Rosie's room, then Sophie walking into Ali and Lisa's room, and both rooms have the very same cracks and stains on the walls revealing it was filmed in the same room, but with different furniture.
Visible crew/equipment: While Sophie's with Lisa and Ali at the dock, after they admire Sophie's engagement ring Lisa bends down to pick up her handbag, and there's a rather large bulge at her lower back under her top - presumably the mic pack, which wasn't there when Lisa lifted Sophie and spun her around.
Continuity mistake: While Ali and Lisa are trying on the dresses for Sophie's wedding, after Sky leaves the room with his props, when Ali and Lisa stand up only one side of Ali's dress is over her shoulders in the shot facing Sophie, but in the shot facing Ali and Lisa both sides are over Ali's shoulders properly.
Continuity mistake: When Tanya is singing "Does Your Mother Know", she walks away from the bar and goes out to the beach to join the crowd of men who are dancing. As she's walking, there's a side shot of her, and in the background you see an extra with a towel around her waist wearing red wayfarer sunglasses. But then there's a face-on shot of Tanya, and the extra's sunglasses have disappeared.