When Jonah is sitting with Jessica in the chair, he tells Sam: "When you play it backwards this says 'Paul is Dead'." The actual saying is 'Turn me on dead man' and it is on The Beatles' White Album, which was released on Apple records. (The record on their record play has a 'Capitol' label on it). The Capitol record that they are playing is more likely Strawberry Fields Forever. In the end section people claimed they heard John sing,"I buried Paul" which contributed to the hoax of Paul's premature death.
At the end of the movie supposedly Sam and Jonah have returned to the top of the Empire State Building to retrieve Jonah's backpack. Yet when they enter the elevator to go back down no one (not Sam, not Jonah, not Annie) has the backpack.
When Annie first starts walking along the observation deck of the Empire State Building after it's closed for the evening, she passes a telescope that has its eyepieces facing the camera. A moment later, as she's walking towards Sam and Jonah, she passes the same telescope, but now it's facing a different direction. That would be hard to do considering that the deck was closed and Annie was the only one out there prior to Sam and Jonah arriving.
On Valentine's Day in New York, while Annie and Walter are having dinner, the sky is getting dark. Then, it is lighter when Sam is leaving the airport. It is darker again when Jonah is shown on top of the Empire State Building.
During the scene at the Rockefeller Plaza when she drinks champagne with Walter, the champagne glasses are not consistent. When the camera is on him, the champagne has bubbles and glasses have condensation. When shots are on her, champagne is flat and there is no condensation.