Would you be able to show how you handle 'on failed purchase' event in your Mortar Melon? I know you did tutorial but you haven't show how you implemented that event.
It looks like you have implemented it correctly there but some of us run into the same problem.
For me the event 'on successful purchase' event also triggers even if user cancel purchase. Strange is that I've tested in test mode VS2013 and work correctly. Not working in store however.
I've described my issue here:
My Construct 2 events code is there as well.