When Amy's parents are talking to her while she eats breakfast, the whipped cream can sitting on the table moves between shots.
After Slappy's head is smashed off, the position of the broken face on the floor changes.
When Sara enters the room after Slappy comes to life after talking to Amy, he laughs but his mouth doesn't move.
When Slappy is about to hit the dad with the guitar, if you look closely, you can see that Slappy is propped up on a stand.
In the promo for the episode, they accidentally said the wrong family name. In the book and episode, Amy is part of the Kramer family, but in the promo, they said the Davis family instead. Slappy's voice is also different in the original Fox Kids promo. In an added bit of confusion, the original Fox Kids promo also refers to the episode simply as Night of the Living Dummy, not Night of the Living Dummy II.
In the long shot of Slappy about to smash the guitar on Amy's father's head, the stand the dummy is propped up on is visible.