About twenty minutes into the film, John steps through the doorway into Pat's apartment twice between shots - and he is holding his hat differently as well.
Near the end of the film, during the rooftop scenes, the bright points of light in some shots, presumably meant to be lights from buildings in the distance or streetlights, move with the camera.
As is not unusual, when Tracy is playing his song and accompanying himself on the piano at the party, towards the end a full orchestra joins in.