They say the devil is in the details—that's certainly the case in Game of Thrones, at least.
If you haven't watched the latest two episodes of Game of Thrones, and if you don't mind discussion of what will probably become a big plot point in future episodes, you should stop reading now: this article contains spoilers.
Okay then. Onto the spoilery talk:
So, the purple wedding. How did it happen? How did Joffrey get poisoned; who did it? Readers of the books might already be in the know, but while the show hasn't outright told us who the culprit is, we all watched the poisoning happen. Most of us didn't realize it, though—but you can catch it if you look closely.
Remember the necklace that Ser Dontos gave Sansa in the start of the season, in that weird scene that seemed to have nothing to do with anything?
The scene is actually pretty important, as is the necklace. Count the crystals. There are seven of them. Okay, now fast forward to last night's episode. Much of the episode slyly focuses on Olenna Tyrell—take, for instance, this ominous shot that happens during the wedding.
Later, after the ceremony is over, Olenna comes over and speaks to Sansa...
She fiddlers with Sansa's hair, she fiddles with her necklace, she offers Sansa comfort. It seems harmless! Except, weirdly, when Olenna leaves, Sansa's necklace only has six crystals—notice that the far right crystal is no longer there:
Where did that last crystal go? Hmm.
Notice how, when Tyrion brings the cup full of wine up to Joffrey, the shot focuses on Olenna again?
That's because she did it. She poisoned him. She asked the Tyrell women to buy the best necklaces in the land, and then presumably gave one of those necklaces to Ser Dontos. Then, when the poisoning happens, we know that the cup was within her reach—she took the poison crystal from Sansa's necklace and put it in Joffrey's wine. That's why Joffrey ends up looking like this:
Not that anyone feels sorry for Joffrey, of course. But the best part about this all? It's that when Olenna takes the necklace from Sansa, she says "War is war, but killing a man at a wedding? Horrid. What sort of monster would do such a thing?"
EDIT: some of you are asking, how can a crystal be poisonous? Well, this is a special crystal—it's a poison called "The Strangler." Here's how it works, according to the Game of Thrones wiki:
The poison is made from plants that are only found on islands in the Jade Sea. The leaves of the plant are picked and aged, then soaked in a wash of limes, sugar water and rare spices from the Summer Islands. The leaves are then discarded, but the liquid is kept and thickened with ash and allowed to crystallize.