Whenever I land a Reverse Falcon Punch in Brawl.
One day, I managed to land a critical hit with Fire Fang and somehow managed to give a Burn Status ailment AND a flinch to the opponent. It was in a Battle Simulator however, and my opponent had ragequitted, calling me a haxxor.
EDIT : Just recalled this now, when I confronted Cynthia in Pokémon White for the first time. My strongest Pokémon was at lvl 55, while her Spiritomb is at lvl 75 in the battle. I managed to PPwaste her Garchomp with my Throh (he's the man of the team) and thanks to my 200+ Hyper Potions and 100+ Revives, I managed to defeat her at last. My reaction after the fight :