The Scott thing. The Syrian government had the rebel issue firmly in hand and was winning so there was no reason for them to use sarin.
Furthermore, Turkey, Saudi Arabia and Israel stood to gain from the U.S. entering the war so they'd have all the incentive in the world to sell
some chemical weapons to rebels, probably the crazier Al Nusra front ones.
The investigation into whether gas was even used or not is still ongoing and the "best evidence" seems to be a fistful of government-supplied
Youtube videos. NATO and the U.N. ain't buying it and the UK explicitly voted against going to war.