^^^ Yup I caught that.
I figured it was Sansa.
The cup fell. And Joffrey kicked it under the table.
But it looked like Sansa handed Tyrion a 2nd cup - which she ready to go, on the floor by her side.

(or she put some poison in the original cup as she handed it back to him)
Plus it makes sense. (to me)
Sansa kills MULTIPLE birds with that one move.
- Gets rid of Joffrey.
- Gets 'poetic' revenge. (it's an Eye-for-an-eye since it happens at a wedding also)
- Gets rid of Tyrion. (because Cersei now has justification to kill him, off pure emotion)
- Hoping it might make the whole Lannister family to implode from within.
If she DID do it... my guess... she got the poison from Lady Olenna. (Queen of Thornes)
And that Court Jester saw it all go down. And knew what was up.
That's why he jumped in to save her... like she saved him.
Because it would not take very long for the Lannisters to figure it out.
Sansa has a bad 'poker face'.
She looked completely 'zoned out' during the whole reception.
She had that "fuck it. why not. i'll do it. let the chips fall where they may." type of expression.