I like this topic, so I'm stealing it for a post here: What films should you not watch until you've seen others that give you the background material? Ones suggested in the thread: "The Great Escape" before "Chicken Run" and the Romero movies before "Shaun of the Dead". (I love "Night of the Living Dead" with ferocity, and I still missed the part in "Shaun" with "I'm coming to get you, Barbara." Auguste had to point it out to me.)
A couple off the top of my head: you should read Emma before you see "Clueless", and you should have a passing familiarity with Anne Rice's wretched vampire stories to really get why "Buffy the Vampire Slayer" was a welcome relief. Tarantino's movies are 100% more enjoyable if you have a good background in trashy B movies. You should see "The Apartment" before you watch the series "Mad Men".
An interesting one from Marc: You should watch "Yojimbo" before you see basically anything with Clint Eastwood in it, but especially "A Fistful of Dollars".
What are your suggestions for the list?