Visible crew/equipment: During the bar fight scene, when the big man picks up Frank a wire is visible on his jacket when the big man throws him.
Revealing mistake: When Alfie (Michael Caine) goes to the dry-cleaners, he starts kissing a woman. He has a grey suit on and he comes out wearing a blue suit, being a comedic trick. But, when the guy in the grey suit goes behind the clothes rail, you can tell it is not Michael Caine, as his hair is shorter and blonder than Caine's.
Alfie: I've never told her that I love her - except at those times when you've got to say something for appearance's sake.