Have you ever noticed that certain movie series' have serious naming problems?
The one that comes to mind most glaringly is the First Blood / Rambo series. The first movie was titled First Blood. The sequel was titled Rambo: First Blood Part II- which makes perfect sense - but then the third movie was titled Rambo III. I guess they just couldn't come up with a title for this one, so they banked on the Rambo name and just tacked on the "III". But it really messed everything up. After Rambo III came out, Rambo: First Blood Part II ("Rambo" for short) became known colloquially as "Rambo II" and First Blood as "Rambo I". What a mess...
The next one that comes to mind is the Star Wars series. When Star Wars (the movie) came out in 1977, it had two words in its title: "Star" and "Wars". Only after its success and the release of Star Wars: Episode V - The Empire Strikes Back did the "Star Wars" title become co-opted into the series name, and Star Wars became Star Wars: Episode IV: A New Hope. From that point on, when someone refererred to "Star Wars", he had to be clear whether he was referring to the movie or the series.
The only other series that comes to mind is The Thin Man series from the 30's and 40's. The problem with it is that in the first movie - The Thin Man - the title refers to a character that appears only in that movie. The next movie's title - After the Thin Man - makes sense - it occurs after the case with the "thin man". But the remaining movie titles - Another Thin Man, Shadow of the Thin Man, The Thin Man Goes Home, and Song of the Thin Man - don't make sense. They have nothing to do with the "thin man", and they mistakenly give the impression that Nick Charles (played by William Powell) is "the thin man", when in fact, he is not.