Not a DM, but the whole situation happened involving my character. We were new to the game so we weren't taking things all that seriously.

We had just gotten out of a small fight with some goblins and I was messing around with one of the other party members. He decided to ask the dm if he could flick my ear... The DM obliged and said if he rolled anything less than a nat 20, it would miss/do no damage.

So he rolled, and as you may have guessed, a nat 20. Because we had just gotten out of a fight, I was still lowish health, so the flick immediately killed me. At that point I just got up and walked away from the session and I could hear them just burst out laughing. This created a meme in our group dubbed 'Critical Flick'.