People are analyzing the show frame by frame. Welcome to the internet.
My point is that particular poster was not. He asked someone else to do it for him to "see if I'm right".
When you're watching the show for the first time, you're not analyzing every frame for clues. It's not a murder mystery. This is Joffrey's wedding. No one thought Joffrey was going to die anytime soon, no one thought anything big at all was going to happen in this episode. So you'd just be watching it like a normal person. I doubt anyone you know who hasn't read the books was saying "why is that old lady talking to Sansa so much" while it was happening -- it didn't seem the slightest bit unusual, she and Margaery have been extremely solicitous toward Sansa since the beginning.
And then your sleuthing would presumably be thrown off by the fact that it makes zero logical sense for Olenna to give Dontos a necklace to give to Sansa just so she can steal it back a day later to poison Joffrey with it. And then bringing Littlefinger into it? The dude hasn't even been in this season yet and neither Olenna nor Dontos have any link whatsoever to him.
I think the poster also referred to it as the "Purple Wedding" already.
It's easy for those of us who knew what was going to happen to talk about how "obvious" the show was.
I guaran-fucking-tee not one person who hasn't read the books noticed a gem was missing on Sansa's necklace on first viewing. It's utterly ridiculous.