Few years ago, playing my Skullbriar deck against a Marath deck of someone that was not very popular at the LGS. I had just enough creatures to block all of my opponents creatures, still live to my next turn and attack for lethal commander damage with Skullbriar (I had
Rogue's Passage). He tapped out to make three 4/4
elemental tokens. Which would eat all my blockers and give lethal damage. He then moved to combat and declared attackers. I cast
Fog (don't remember why it was even in the deck, think it was just a placeholder). In response he removed counters from Marath to deal lethal to me. I said gg and got up to go
smoke. As I was walking out the door someone pointed out that he didn't have the mana open to do so. Since we had already picked our cards up I was just like whatever, gg, we'll play again when I come back in. He packed up and left while I outside despite having said he was there for another few hours.
The misplay was not paying attention to what resources he had available, and probably not my worst misplay but one that still bugs me to this day as he bragged for the next six months how "he cleanly beat me by outsmarting me".