When Charles Bronson reads the lips of the men in the park, he hands Jan Michael-Vincent a tape recorder and tells him to turn it on, ostensibly to record what he is reading. However, he only pushes "Play", not "Record".
The Fiat car used by the Mechanic in the case in
Naples area is blown up. However the car is used again a few minutes later. Not only is the registration plate the same, it still has only one brake light working.
At the end of the movie, when Steve takes the taxi to Arthur's house, you can see Steve's car and there is clearly no note attached to the rear view mirror. When Steve leaves Arthur's house and goes to his car there is a note on the rear view mirror. This should have been visible from the taxi.
As Bishop browses through the secret dossier on himself that he finds in Steve's desk, one of the papers shows his (Bishop's) age to be 44. But it is very obvious that Bishop is considerably older; in fact, Charles Bronson was 50 or 51 when this movie was made.