Continuity mistake: When he's called in to see the Captain, Montag wears his wristwatch turned inward (with the face resting over his pulse point). Seconds later, though he hasn't touched it, the watch has rotated itself 180 degrees and is now on top of his wrist. (00:26:05)
Revealing mistake: Near the beginning, Montag puts on the fireproof clothing to use the flamethrower. It's really footage of him later taking off the same equipment played in reverse.
Visible crew/equipment: Near the end when Montag arrives at the Book Peoples' commune, he is greeted and ushered into a house. There's a shot from just inside the building, looking out a window towards Montag and his host as they approach. As the POV shifts a little, a reflection of the camera becomes plainly visible in the window glass.
Deliberate mistake: Montag's hair gets intermittently shorter, longer and shorter again (and acquires a "cow lick" that wasn't there before) near the end of the film. (Oskar Werner had clashed with Francois Truffaut and deliberately had his hair cut to spite director.)
Revealing mistake: The first time Julie Christie is seen in the monorail the window behind her has artifacts from the matte effect.
