After Quinn is dragged down the waterfall, he says, "I'm fine." But his mouth movements don't match the words. He also says something about it being her turn, which is also not synchronized with the audio. The mouth movements he is making for both sentences are simply him drawing breath.
Robin is wearing a long white dress when they crash. She changes into floral shorts and a tank shirt when she and Quinn go looking for fresh water. After he gets the snake out of her pants in the water, they cut to next scene where they are back to the beach where the plane is. Robin is looking for fresh clothes in her suitcase and the shorts and tank shirt she was just wearing (which had gotten dirty and wet) are in her suitcase folded and clean, and Robin is changing out of the white dress she had already changed out of.
When the pirates' gunboat is attacking the plane, it is initially shown moving towards the beach at some speed. When the plane takes off, however, the boat is seen to be perfectly still with no visible wake near it.
At the airport in the final scenes of the film, when Quinn gets out of the helicopter, his shadow is very short from the sun which is almost directly overhead. However, when he meets up with Robin again, the shadows are much longer despite only a few minutes having passed.
During the scene when Robin inflates the raft inside the plane the plane's flaps are in the full down position, but when Quin goes around the plane to talk to Robin with her face pressed against the window the plane's flaps are now in the full up position.
In the beach scene when Robin hears/sees the commercial airliner and tries to signal it with the flare gun, she struggles with Quinn for the flare gun because he was passed out. Initially there is an open bottle of liquor between Quinn's legs. After Robin gets and shoots the flare gun Quinn wakes up and rolls over and the bottle is now under his arm.
Robin and Frank are seen arriving and departing Macatea on Aloha Airlines, but the airline never operated anywhere near Tahiti. Before ceasing passenger operations in 2008, it only flew between the Hawaiian Islands, the west coast of the US and Canada, and one weekly round trip to Christmas Island in the Gilberts.
While overlooking the coastline high atop a cliff, Quinn is talking to Robin and you can see a tour helicopter flying behind him. The coastline they are overlooking is the Napali Coast which is a popular destination for tour helicopters on the Island of Kauai, Hawaii
After discovering the Japanese float plane, Robin is awakened to the sound of Quinn sawing the floats off the plane. He is using an apparently new hacksaw, which he did not bring with him. Did he return to the crash site which was several miles away? Or did he find one inside the float plane? If indeed it was inside the float plane, the blade would have rusted through after 40 years in the jungle.
When Robin and Frank first arrive at Macatea there is a beautiful view down the beach. This is exactly the same shot (including the tourists, furniture, clothing) that appears in the brochure Frank showed her in the restaurant in New York.
At the end, when Quinn goes to Robin arguing with the custom agents, the passengers in the background are climbing the steps to get onto the plane. But while they are talking the passengers are seen disembarking the same plane in the background.
After Quinn and Robin fly the plane back and crash into the harbor, there is camera equipment visible attached to the underside of the right wing. You can see it behind Robin out the window as she is assessing Quinn's condition and says, "We did it." It looks to be cabling of some kind, and has a bright red tag attached to it. When Robin is being helped from the cockpit a few scenes later, the equipment is gone.
When landing the plane at the end, from the cockpit it looks like the plane is about 100+ feet off shore when it crashes. However, when the fiancé runs out to get his betrothed out of the plane it is only ~20 feet off shore.