Warning, people: the spoiler gate has been lifted, and they're starting to flood through. I was on the Witness subreddit on a speculation thread about what kinds of puzzles that we thought would be in the game, and someone shared a solution to a puzzle that was both NOT speculative and, as far as I know, NOT revealed elsewhere. Granted, it was covered with spoiler tags and started with "we know that" as if it were common knowledge, but it most assuredly was not.
So if you're planning on playing the game unspoiled, it might be best to step out of game discussion right around now.