This post would be more relevant if I wrote it thirty years ago rather than now, but the Internet wasn't much of a thing back then and I only just stumbled across Murder on Flight 502 again...
My favorite comedy for much of my childhood and Airplane (1980). The absurdest humor was like nothing like I'd seen before, and it scratched me right where I itched. It was 15 years before I discovered that most of the scenes and characters from the movie (and the other Zucker-Abrahams-related movies) were send ups of specific scenes and characters in other movies (movies like Murder on Flight 502 and Airport, Airport 1975, Airport '77, and Airport '79). Now knowing that there was now so little about the movie which was truly original, aside from turning each element on its head, robbed me of most of my appreciation for and enjoyment of the movie. Part of what impresses me about stories is the author's selection of characters and events, why did they decide to add this character to the story, why did they give him the personality they did, why did they inject him into this particular interaction with another character. So suddenly finding out that most of those decisions were copied from other writers made Airplane seem a lot less impressive a creation. It's still funny, of course, but it feels now funny in a more Mystery Science Theater 3000* sense of just adding jokes on top of something rather than creating an entire world of new, funny material.
* I've always wanted to love Mystery Science Theater 3000, and I've watched dozens of episodes, but I watch them for the B-movies they show and seem to tune out the running commentary, which I don't think has ever produced even a hint of a chuckle. I wish I could laugh at it, I honestly do, but it's just not my sense of humor, I guess.