The Hobbit: An Unexpected Journey (2012)

Continuity mistake: In the scene where Bilbo, now invisible, sees Gandalf and the dwarfs run past the cave and he can't join them because Gollum blocks the way. When Bilbo finally runs past Gollum and kicks him in the face and knocks him down, the very next shot shows Gollum's body lying in the opposite direction, away from the direction Bilbo kicked him. (02:19:35)

Bilbo: Good Morning!
Gandalf: What do you mean? Do you wish me a good morning, or mean it is a good morning whether I want it or not; or that you feel good on this morning; or that it is a morning to be good on?
Bilbo: All of them at once.

Trivia: The blade that Gandalf presents to the high Elves and Saruman is the same blade that Frodo gets stabbed with in Fellowship of the Ring on Weathertop. Confirmed in the director's commentary. How the blade makes its way back to the witch-king hasn't been revealed.

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Question: Was any reason ever given as to why Gandalf chose Bilbo to go on this journey and not someone else?