Frank's hair changes to more slicked back when talking to Sonny in prison.
When Stan is scanning and submitting the pictures, the feedback on the computer screen misspells the word "asynchronous".
In the opening scene the killer racks the slide on his shotgun at the front door. No cold blooded killer would wait until they were at the targets house, to charge their weapon. One it might alert the target and no professional would ever carry a weapon chamber empty.