Contains spoilers for The Sixth Sense! If you haven't seen the movie yet and don't want to be spoiled, stop reading now!
I doubt it was the first story to employ the device, but The Sixth Sense is the first movie I recall watching where we're led to believe that a major character is alive when he is in fact dead. It's gotta rank up there with the biggest reveals in movie history alongside Citizen Kane, The Empire Strikes Back, and Psycho. The idea that a character may be dead is now something I look for all the time in suspense/horror movies, and it's led me to correctly guess what's going on in at least two movies (I won't name which ones so that I don't spoil them).
Lately, the same thing has been happening with books I read. Just in the past two months, I've read two stories where at least one of the important characters is dead. In one case, I wasn't looking for a twist, so it caught me by surprise. In the second case, the book description reveals that there's going to be a twist (a bad idea IMHO because that's a hard promise to keep), so I tried to figure it out as I read the story. Sure enough, not even halfway through it, I started picking up the signs that certain characters might not be alive. I noticed how other characters didn't interact with them, how they were described, how people reacted at the mention of their names, etc. So when the supposed big reveal came at the end, I had already guessed what was going on, and the book was ruined for me. (There were also other aspects of the novel that annoyed me, so I was waiting for the twist ending to save it).
Am I going to stop wondering if so-and-so is dead when I watch a movie or read a book? Probably not. It is kinda fun to try to figure these things out. However, now that The Sixth Sense planted the idea in my head, I can't shake it.