I was the cause of a TPK as a party member. I am notorious for my bad rolls, to the point it made my character useless and myself as a player unable to really involve myself in any rolling interactions. So to combat this we homebrewed a character for me that was a sorcerer who was obsessed with the arcane and made a warlock pact with a chaos demon.

This pact gave me great power at a cost (Meta >3 was double damage 4-20 1/2 damage, and I roll for my spell selection each day, we had a chart, DM approved. I also rolled each cast to see which of a type of spell I'd cast). While the cost was great I also became insanely powerful, unexpectedly so, by everyone. Eventually the party and I were told about a house that people kept disappearing into and how it used to be an old brewery in the basement before the owners just disappeared too.

The party set off! We make it to the brewery a f as soon as myself and another party member set foot through the door some goopy slime falls onto our shoulder. Confused we look up just in time for a massive gelatinous cube, fed by many town people, to envelop us. The monk just behind us goes to start wailing on it as his two party members are helplessly engulfed but stumbles on the entrance and is equally absorbed.

I, a great wizard with a penchant for the insane arcane, decide I shall boil the gelatinous cube. I hope to roll a lesser spell but end up with the only high(er) level spell in my damage selection. Fireball. Roll my homebrew damage mod hoping for 1/2 damage, 2. Double damage fireball. Point blank. Inside a confined (gelatinous) cube. There were no survivors, but great stories never die.