I gotta disagree with DomA, because I was watching for that stuff, and didn't see it. In the books, she was the one who poisoned the cup. Dontos first came to Sansa shortly after she saved him from Joffrey. He was her confidant the whole time, and was the one who left slip the plans to marry a Tyrell, who told her to bail during the siege and hide in her room, and after Joffrey put her aside, he gave her a hair net to wear at Joffrey's eventual wedding, telling her it was vengeance for her father, that it was a magic net that would take her home. During the wedding, Olenna fussed with Sansa's hair a bit, and afterwards, Sansa noticed a gem missing from the net. Later on, it is revealed to Sansa by one of the conspirators (who was not at the wedding) that Olenna's fiddling with the net was the ruse whereby she took the poison hidden in it to slip in the cup.