One of the characters turns out to be a dragon in disguise.

The hero turns out to be the villain and he knew about the dragon which he has sworn to kill over some long ago act the dragon committed even though it means the end of the world as the characters know it.

The heroine turns out to know that the hero is a jerk and does her best to protect the dragon.

Or the party discovers and artifiact that summons Ctuthulu.... let the mayhem ensue!