The hand-held device that controls the mechanical bed has a red button on top and a green button on bottom. But later, when the killer is pressing the controls, the stop and start buttons are in opposite positions.
The goof items below may give away important plot points.
The first murder committed, that of Paul's fiancee, shows the murderer holding the knife in the left hand throughout. However, the murderer turns out to be Herbert Flay, played by the right-handed Peter Cushing.