A glitch...

In the celebrated shot(s) where the camera pulls away down the stairwell of Rusk's apartment building and out into the street after bringing in Barbara, the paint (or lighting) in the hallway doesn't match.

The walls are a bright white in Part 1 (i.e., the soundstage set) but then appear more aged and yellowing in Part 2 (the actual location, after the man passes the camera with the sack on his shoulder).

I wonder why the production team wasn't more careful about this. I can't say the difference jumped out at me the first time I saw it, but I imagine this would have been even more obvious on the big screen. (Perhaps the negative has aged oddly, though, and the difference wasn't originally so great.)