WARNING: Ancient Greek Nerd Attack!
You know what kind of bugs me? Trilogies. It bugs me when a movie/game/what have you is so popular that it gets two sequels, and then everyone pretends its a trilogy.
For example, take the Toy Story movies. They made one in 1995, one in 1999 and one in 2010. Clearly, the three movies were not made together. But people say it's a trilogy anyway, just because they're three of them.
That bugs me. A trilogy is different from a series with three separate installments. A trilogy is a series with three unified installments, specifically designed to go together.
Sometimes, I think that the only legitimate kind of trilogy is something like Aeschylus' Oresteia, a series of three plays about Orestes. The three plays were performed once, all on the same day. There was no way to see one play separately; you either saw all three or none of them, because they go together. Now that's a unified trilogy.
I don't think we have any trilogies today which are unified like that. Even with trilogies like the Lord of the Rings movies, the three installments were released separately, not all together. Seeing the movies as a group was not possible; people were forced to see them separately. That's very different from the Ancient Greek scheme where you had to see all three plays at once, because they were not performed separately (or multiple times).
The sad part is that most of the Ancient Greek plays got lost over the past few thousand years, so we no longer can enjoy them in the original trilogy format. For example, you can't read the Prometheus trilogy because we only have one of the three plays, Prometheus Bound. That's really unfortunate, because I want to read what happens in the next two plays of the trilogy.