See the article below. I've heard echoes of this line of thinking in the media all last week: Why would the Syrian government use chemical weapons so openly, indiscriminately and so close to Damascus when UN investigators, experts on the subject, were there and while the government forces, by most accounts, have the upper hand against the rebels? That's not very smart.
On the other hand, is it credible the 'rebels' did it to themselves to provoke global outrage and a military response? Sounds ridiculous on first glance but....
a. the rebel side is fragmented into groups, some with very extreme ideologies, some from outside of Syria who wouldn't hesitate to kill local civilians to gain advantage
b. is it conceivable rebel groups could have captured chemical munitions to use in such an attack?
c. is it conceivable that one extreme rebel group could have staged the attack and the victims, and other rebel groups, not even know it was staged?