I'd love to do a team comic with some of the under-used DC characters. Creeper, Man-Bat, Vixen, that latina Wildcat from the late 80s, and maybe Etrigan the Demon as the leader. Give it a nice street-level occult vibe, and a slightly weird name like "Hand of Glory"....
... well, I'd buy it.
Guess what? It's been done before. No, not literally, but there are more than enough similarities that all it takes is one glance at my proposal to go, "Oh, that sounds like Shadow Fighters." Maybe you can't see it, but I can. And now I can't use that idea, because I refuse to write anything that could be construed as ripping off another series.
And then over in Plok's blog, he challenges us to pitch a Time Travel TV series. My early contribution:
The next day, I find out that it's essentially the premise of Voyagers! done with the cast of Oh My Goddess!
I'd kill myself in despair, but that's been done before, too.