Sadie Mae Scroggins' coffee mug disappears when she goes to dance with Shaggy.
The type of gun Billy Bob uses, in reality, can only shoot one bullet at a time, unlike in the movie, where he shoots multiple bullets from it at a time. As neither Billy Bob nor his gun have any ghostly attributes, they should be bound by realistic physics, regardless of whether it's funny.
The phone book ad for the Boo Brothers says "...special this week, three ghosts for the price of one." Real phone books are only issued once a year, not every week. However, a ghostly phone book is not necessarily bound by these rules.
The Boo Brothers are ghosts that can go through objects, walls, and other obstacles. Several times they get smashed, stuck, and squashed when they should have just went right through whatever they ran into. As they are fantasy characters, their abilities and limits are whatever the filmmakers say they are, even when they are apparently contradictory. Furthermore, the convention of animated comedy is that anything can happen as long as it is funny.