During both when Debbie is watching the crime investigation show and when Joel is showing Wednesday and Pugsley his serial killer cards, it states that the Black Widow has killed three different rich husbands. But later when Debbie is showing her slideshow to the Addams family, she only accounts for two.
When Debbie is speeding in her Mercedes after Fester and Thing, she nearly rear-ends a Lincoln Continental. In the next scene, she's driving behind a completely different car and the same Lincoln she was behind can be seen driving the other way down the road, and she nearly hits it head on.
Wednesday and Pugsley attempt to drop an anvil suspended by a rope on Pubert at the base of the staircase before Debbie removes Pubert from harm. A following shot of the staircase with Gomez and Morticia descending shows the rope is suddenly gone.
When the back of the "Murderers Killers and Pyschos" card is shown, it contains a typo in the biography that states "Known for marring rich lonely men and then killing them on what hey (sic) expected to be a glorious wedding night, the Black Widow uses a variety of disguises to lull her victims into a sense of false security. Her exact likeness is still unknown to the police."
When Wenesday and Pugsley are about to use the guillotine when playing with Pubert, Wednesday rolls up her scroll during her line "Justice is served" right when the watermelon is chopped in two. In the next shot, it appears as if the scroll is back to being unrolled and she rolls it up again during her line: "Bring forward the evil one". Close observation reveals that Wednesday instead rolls it back in reverse to the first time.
When Gomez goes to the police station to report his brother Fester being held captive by his wife Debbie, the Desk Officer (Nathan Lane) has Sergeant stripes on his uniform sleeves, but a patrolman's badge (silver) on his shirt. While Sergeants wear gold badges in many Dept's, not all Dept's do. In many small communities, only command officers (lieutenants, captains, chiefs) wear gold badges.