If you've read the books, cool, but NO SPOILERS. KEEP YOUR MOUTH SHUT.
Also, Sersei is about to bring the pain.
It wasn't a random guy that told Sansa to leave... it was Ser Dontos. (He was the one who showed up drunk to the duel on Joffery's name day, and Sansa saved his life. Joffrey made Ser Dontos his fool instead of killing him.)
In the most recent episode, Ser Dontos gives Sansa his only remaining valuable possession, the necklace with blue glass/crystals, which she wears to the wedding. He basically thanks her for saving his life and indicates that he has nothing left to live for.
When the shit starts to hit the fan, he urges Sansa to leave before it's too late, so I think he poisoned Joffrey.
Although if that's true, we are robbed of the joy of one of the main characters having the satisfaction of killing that sadistic little prick.