When Mason walks into Moxley's house and walks up the stairs, there is a beautiful French window behind him, but when his car pulls away later, as seen from the outside, there is no matching French window.
When Carl Montaine throws the chair at Moxley, it breaks a mirror and most of the glass falls out of the bottom half. In the next shot though, a lot of the glass reappears, and the mirror's glass is broken much differently.
Della is holding a telegram in one hand. When Perry says "Della Street", Della is holding the telegram with both hands.
When Mason and Spudsy arrive at the Moxley residence by car, the camera and cameraman across the street are shadowed on the side of the street car which crosses between.
When Mason leaves the table at the restaurant to go to the wine cellar, a moving shadow of the boom microphone is visible upper right of the frame.
When Perry is picking out crabs at Fisherman's Wharf, he says, "Joe, we've decided upon these four antagonistic anthropods." Crabs are arthropods; there is no such thing as an anthropod.