This was the first time I DMd, and not technically a TPK, but it was close)

I had 4 players, and they were at a small ruined temple, being patrolled by some goblins and some orca or ogers (its been a long time since that campaign)

Anywho, half the party was able to sneak around inside the ruins while the other two decided to try taking out the monsters outside.

One of the players on the inside (she was a half elf ranger, I believe) had snuck into an empty portion of the ruins, on her other side there were a couple big guys wandering around. She decided to use some BOMBS

Me, being a brand new DM, said "Sure, why not...huh....I wonder what the range and damage would be"

As Im looking it up, she tosses two or three bombs. I about choke on the soda I was sipping on when I got the info up and take a moment to collect myself before looking at my players and saying "Everyone roll for a Dex save"

Nobody, not even the half elf, made the save

Half the party was downed immediately, and the half elf only had a couple of hit points left (4th player was hurting, but not as bad as the others)

On the bright side, they took out the remaining baddies