Basically, this happens when the numbering of sequels gets really, really complicated. A Sequel Number Snarl often occurs when Sequels and Interquels start filling up the chronology, but then gets fouled up by the presence of an Oddly-Named Sequel 2: Electric Boogaloo or two (or five...) whose title doesn't include a number, is added to the series. A subtrope of Numbered Sequels. Examples of Sequel Number Snarl include: