Just played a game where on turn two I had one forests in play, a forest in hand, and a Llanowar Elves played on the previous turn. I cast a Sazh's Chokobo before playing my land for the turn to get the landfall trigger, and an Innkeeper's Talent in hand I wanted to play after. With my Chokobo on the stack my opponent kills my Llanowar Elves. With just one tapped land I "technically" didn't have enough mana sources on the battlefield to cast anything in my hand, so I wasn't given a window to tap my Elves in response even though I am supposed to get priority to respond before my opponent's spell resolves.