There are a number of misplaced "items" in the movie. Olivia Wilde's character is supposed to be in Paris, but she can briefly be seen in the back seat of a car driving by the Bar Americano. One of the paintings in James Franco's studio is on display in Rome, but with a different signature. The obvious one where the note Mila Kunis writes in the New York hotel is picked up by Olivia Wilde in Paris.
The full name of James Franco's character, Richard Wiess, only appears twice and is never spoken. You see it first in Paris, when Michael (Liam Neeson) and Anna (Olivia Wilde) are walking in front of a building (you can see that his work will be featured there soon), and again when Julia (Mila Kunis) rings the buzzer at his building in New York. The name next to the buzzer is R. Weiss.
James Franco said that Ashton Kutcher, Mila Kunis' husband (fiancé then), couldn't endure watching the scene where Franco had to slap Kunis' face and drag her across the floor. Kutcher had to leave the room during the shooting. "I mean, it wasn't my idea!! It was the script!" Franco said.