I shot some footage of a subject supposedly on a subway, holding a vertical pole to maintain balance as the sub moves. After keying the footage and getting the color right, I noticed that although the footage is perfectly still -- subject was standing in place, and camera locked on a tripod -- after inserting the footage into the sample background, the subject "floats."
The entire footage moves around a couple pixels left or right or up and down as if she's floating.
This is the first time this has happened. I have shot footage of people walking or moving about, and when I compose the scene, as long as I get the angle of the flooring correct beneath the actors, it appears as if they are walking, and their feet do not float or glide along the floor.
This time, however, that's exactly what happens although the subject stands still.
Are you completely sure your background is not the problem? I got a stock sequence (lockdown) just the other day that was messing with my head because the camera wasn't stable as the shot started (probably from pushing that little red rec button!)