Revealing mistake: When there is a flashback of Thorin in the war, in the first few seconds, on the left of the screen, there is a man with a shield. He swings it at an ogre and you can see it misses by a good five inches, but the ogre moves his head back as if it were hit.
Revealing mistake: When Gandalf marks Bilbo's door, you just see the outline already marked on the door. (01:15:10)
Answer: Gandalf has already convinced the dwarves Bilbo is a burglar and Bilbo realised he needs a lot to convince them otherwise. Bilbo is also intimidated by Gandlaf and confused about his claims so he is not sure how to handle the situation.
lionhead