Lofty Denial with Riddleform in play
I had a Riddleform in play but no creatures with flying. I played Lofty Denial to counter my opponent's spell which triggered my Riddleform making it a 3/3 flyer before my Lofty Denial resloved. My opponent still just needed to pay 1 mana to fizzle my Lofty Denial even if he should have needed to pay 4 since I had a flyer in play. Last game in my log file was lost due to this.