Yeah, Tv Tropes will do that to you OP. Strangely though I don't tend to get this too often, or if I do I hardly notice or care, even if I end up reading up about stuff on tv tropes first (which I tend to do if I'm in the middle of a series which I've recently become obsessed with).
It kinda happened with the Buffy comics season 8 (not due to tv tropes, merely by me looking up the names of the compliation issues I was buying so I could find the next issue) with regards to the Big Bads identity. More recently I had it happen slightly with a few small plot points in w.i.t.c.h, but it wasn't too bad.
Somehow I tend to avoid most bigger spoilers, like the identity of the origama killer from Heavy Rain, everything that happened in Infamous (still haven't played 2, know idea what happens still), the "Would you kindly?" thing, every revelation in Lost and everything from Portal 2, despite not playing it until several months after release.
I even managed to avoid the ME3 ending spoilers, though I knew people were complaining about them I somehow managed to avoid direct spoilers.
So er, go me I suppose!