When Tony is sitting with Carmela on the sofa after Janice goes back to Seattle, he says to her "..after 18 years of marriage, don't make me make you an accessory after the fact", you can clearly see that Gandolfini said something else, and the "18" was dubbed in. Obviously, either Gandolfini forgot the line, or, more likely, they changed the number while editing.
The goof items below may give away important plot points.
While Christopher and Furio are cutting up Richie's body in Satriale's pork store, after Janice shoots him, you can hear the sound of a saw blade running and something being cut, but it is obvious that the blade of the saw is not actually moving.
After Richie punches Janice and sits down at the table, we see the back of the wine bottle and basket with the bread is horizontal on the table, then the frame is showing Janice, then its back to Richie, we now see the bottle's label and the basket is vertical on the table, then Janice walks away, we see Richie eating, and the we see the back of the wine bottle again, Richie grabs the wine bottle to pour some wine, a 2 seconds later we see Janice pointing the gun at Richie and his glass is full of wine.
When Janice gets on the bus near the end of the the episode the camera pans left revealing what looks like a member of the crew moving alongside the camera as it pulls forward. Members of crew also visible in the reflection at the end of the shot.