In a scene near the end, Julia Roberts goes to straighten George Clooney's tie and his jacket is unbuttoned. In the next shot it's buttoned, and in the next it starts unbuttoned and he buttons it in the shot.
When Lily climbs aboard Gede's boat after she and Wren were left stranded in the ocean, she is no longer wearing her snorkeling mask nor is she carrying it.
Lily is supposed to be starting work as an attorney after she returns from Bali, a trip she departed for immediately after graduating from college. However, in the United States, attorneys (in most circumstances) must attend three years of law school after college and then take the bar exam in the state they intend to practice in; furthermore, the bar exam is only given in July and February. She would most likely be too busy studying for the bar to take an extended trip to Bali.
When David and Georgia check into the motel, he is assigned room #221, and she is assigned room #222. When they go to their rooms, he enters room #222 and she enters room #221. However, this is because Georgia switched rooms because she thought David had a better room (she says "If I'm going to be listening to you (snoring) all night, I'm doing it from a better room").
When Georgia and David are dropping Lily and Wren off at the airport (that is in the United States), a sign references "prams" and "trolleys." In an American airport, it should read "strollers" and "carts."
When Lily and Wren first arrive in Bali and are swimming/freediving in the ocean, they surface to find their boat has left and gone back to shore without them. But they are both only wearing goggles, not scuba diving equipment (or even a snorkel) so could not have been under the water for longer than a minute or so without coming back up for air. This would not give the boat time to disappear completely from view.
The day before the wedding they go to a remote beach but Georgia (Julia Roberts) initially decides to stay behind to care for Paul. When the party reaches the top of a hill to enjoy the viewpoint, Georgia just catches up. So either she found a chopper (which silently dropped her in the jungle) or someone gave her a ride to this remote part of the island and then she found her way running crazy fast through the jungle.
Julia Roberts' character states she was headbutted. George Clooney's character says "Let me see your nose." Her nose was hit, but she hadn't mentioned that information.