How did Olenna slip the poison from Sansa's necklace?

The gems in Sansa's necklace are actually poisonous. In the books it was a hairnet. Ser Dontos gave Sansa the necklace to wear, ensuring she'd wear it at the wedding. Lady Olenna comes over to make small talk with Sansa and adjusts the necklace, nicking one of the stones in the process. read more

Since Sansa was wearing the necklace throughout the wedding & nobody but Olenna goes up near her for a small talk, there would be no time to mix the poison in the pigeon pie (cake). Hence, conclusively & certainly, the poison was mixed in the wine. read more