The bayonet mount on the body could be loose or maladjusted.
The mirror could be loose or maladjusted. (This happened with one of my F-1s right after a CLA, and it ruined 90% of the shots on about 20 rolls of film. It is not easy to notice the problem unless you focus on infinity or use your distance scale a lot, and with my 50mm lens, I rarely do that, so I did not notice until near the end of a long trip.)
The focusing screen could be loose or maladusted.
The focusing collar on the lens may not be moving the internals of the lens to the proper position. (My Canon 28mm f/2.0 had this problem. When the collar was set to infinity, the lens was not actually focused on infinity.)
Only the latter is an issue with the lens.