Ideas that you were beaten to/weren't aware has been done.
Ever had an awesome idea for a movie/game/TV show/comic/invention, only to realize that someone has beaten you to it or it has been done before?
I had an idea of a movie where a guy journeys from heaven to hell to rescue his wife. I was shocked and saddened when I found out that it was a novel AND a movie adaptation already.
My Friend: You know, if only Bruce Wayne was bit by a radioactive psychiatrist rather than a bat, Gotham wouldn't even need Arkham Asylum.
Me: Haha, yeah. That's true.
Me: Wait, Bruce Wayne was never bit by a radioactive anything.