The color yellow is used prominently throughout the film. It represents sunshine, happiness, loyalty, joy, optimism, remembrance and warmth. It energizes mood and relieves depression. It's the lightest and brightest color on the color wheel. It's an attention getter - one reason taxi caps are painted yellow and in most countries traffic lights and signs are yellow, meaning caution. Even some blind people can detect the color yellow. On the other hand, it represents cowardice and deceit. Judas is often depicted wearing yellow.
The trivia items below may give away important plot points.
In an interview with MTV.com, director Marc Forster revealed the reason why Sam's trousers are too short for his legs. It is because Henry's view of Sam is when he is crouching down, and his trousers pull up when he does this.
When Sam leaves his apartment and Henry is waiting outside, as they walk downstairs the room they stop in front of to talk is number 21. At the end, when the camera is panning up away from the scene of the accident, the number on top of the ambulance is 2121. They also live on the 21st floor; the sign is visible when Sam and Lyla discuss trust after he confronted her about not taking her medicine. This is in reference to Henry's 21st birthday, the recurring theme.
The original ending of the movie had Henry standing on top of the bridge (where he is last seen with Sam before the "truth" is revealed). He is shown as gray and spirit-like, looking down on the events that are happening on the bridge.