When Mr. Ciavelli was inside of a long narrow tunnel. He chisels through the thin brick wall of the tunnel and goes inside of a room, and along that same wall is a door leading upstairs. On the other side of the wall, there was no evidence of a door.
When the police enter Gus's diner, he turns the volume of the music up, but oddly he does it by turning the dial anticlockwise, which would normally turn volume down. When they go, Gus turns the volume down, again turning the dial anticlockwise.
The goof items below may give away important plot points.
When the police break into what turns out to be Louis' memorial service, the door swings open all the way and sounds as though it hits the wall behind the door. In the next shot, from the inside of the room, it is clear that there are people seated behind the door who should have been hit by the door crashing open.