Richie punches Janice in the mouth hard and she starts to bleed, when the camera goes away from Janice then back, there is no blood anywhere.
When Tony pulls the slide back on the semi-automatic pistol while exiting the car outside of his sister's house, it is evident that there is no cartridge in the weapon. But even so, the slide cycles normally to the forward position. If there was an empty magazine in the pistol, the slide would lock back, but it does not. This would indicate that a magazine was not inserted into the weapon. This would allow the slide to operate normally, even absent a cartridge being fed into the chamber.
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.
In the seconds right after Janice shoots Richie in the chest while he is sitting at the table, and he starts leaning back on the chair, about to fall, there is a crew/assistants hand visibly grabbing the chair from underneath the table, and forcing it back while Richie is seen struggling to hold his balance, ultimately falling back from the chair.
Tony arrives at his sister's house and is seen taking the actions that would chamber a round into the pistol (although there aren't any cartridges in the weapon - discussed in revealing mistakes elsewhere). There is no reason that Tony Soprano would be out and about with a pistol that is not loaded, as he would understand that when a weapon is needed in the circumstances that he would often need one, having to chamber a round could be the difference of living and dying. Taking the extra couple of seconds to load a gun would not be the actions of someone in this position and appears to be done just to emphasize the fact that he has a weapon.
At 20:00, when Janice is talking about Richie using a gun during sex she says that it is OK because he takes the 'clip' out. Pistols use magazines, not clips. When shown previously where this occurred, he is using a revolver. Revolvers do not have magazines.