Originally posted by joe1955 A perceive parallax error may be caused by the mirror not returning to the 45° angle. If this happens the image as viewed in the viewfinder will be either higher or lower than the image viewed on the print.
All true.
Just as a momentary lapse in Einstein's theory of General Relativity is potentially an explanation of any number of puzzling gravitational anomalies.
However, it remains the case, that an actual example would be most helpful in ascertaining whether what has been observed is really due to the alleged cause.