It feels like when I land on floor that has just scrolled offscreen and jump to save myself, my double jump doesn't work. It's a little hard to say - maybe when falling I jumped again before touching the ground, and spent my double jump. Still, quite frustrating, because that's when I really need the second jump...
@dan_t: Zero is natural according to some conventions, including the mathematician convention. Not sure which convention CS people use, but zero natural is not incorrect, just ambiguous.
 "Diamond Hollow"){kind=icon}
 "Manufactoria"){kind=icon}