As the number of films in a series swells, the probability of an entry that is unmitigated crap jumps to a number greater than 50% with the second installment, and approaches 100% thereafter. For example, a successful film has a sequel that is decent but nothing compared to the original film. Suddenly a third movie appears and is may not even be produced or directed by the same people as the other two. If there are any more sequels, it just goes downhill from there if none of them are entertaining. Do note also that Sequelitis can and does incorporate several of these types at once.