There wasn't anything really tipping people off, it was meant to be a surprise. I just approach it with this question. If I was writing this, who would I make the surprise villain? It works most of the time. I'll have to watch it again to see if anything tipped me off.
Shutter Island was the funniest one to happen with me. I just jokingly said,
. This was in the first five minutes of the movie while they were still on the boat. It was a joke, I wasn't serious, but I was right.