I've got an interesting TPK story for you, featuring a 5th level party decimated by ... 1 troll.
Our group was essentially hit with a random encounter. One of those "you're taking care of normal business in town when you hear screams from down the street" situations.
Three of us began making preperations while the headstrong ranger moved out to investigate. Some troll-lings (small baby trolls) were running around causing havok. The ranger engaged them while the other three of us kept prepping. We thought we were in good shape... we had acid and fire, full hit points, the works.
When the troll mom showed up, we started laying into her with spells and weapons... or so we thought. Every time the damage came up for a vial of acid, it was a 1. Every alchemist's fire rolled a 1. Melf's acid arrow did a total of 5 points of acid damage. Meanwhile, the troll is rampaging through the party. I was playing a gnome wizard, and when the troll attacked me I thought I would be mostly okay... I had the gnome bonus vs. giants, mage armor, shield, and a dex bonus helping me out. I actually held her off a couple of rounds, but the troll made a lucky grapple attack and suddenly I was merely a snack. While I was dying, the fighter was rolling nothing higher than a '4' to hit and cursing the dice gods mightily. The cleric darted in to try and heal my wizard, thinking his 22 AC would surely protect him from one AoO (due to the Troll's reach). The DM rolls a 20, confirms the critical, and the priest joins me in death (he was actually at -9). The Ranger (miraculously unhurt during the battle) chooses not to use the wand of cure light wounds to heal the cleric (settling his fate as he failed the 10% chance to stabilize and died) and instead kept firing arrows at the troll. Arrows from which it was regenerating every turn
The fighter had 20 hit points remaining and tried to cover the Ranger's retreat. The troll hit with both claw attacks, thus gaining it's Rend damage... for a total of 36 hit points!
The Ranger fled the scene and we had a TPK. Our new characters have had one game after this so far.
What's funny is, our party did just about everything right. We had solid tactics, the right gear (including fire and acid), and were freshly rested for the battle... what killed us was the DM's dice rolling. Lot's of 19's, a very telling critical, and our dice wouldn't roll above a 1 when it came to damage the troll couldn't regenerate
There wasn't any one thing you could point to and say "that's where we screwed up", it just happens that way sometimes. TPK's, like a four-letter-word I won't say here, happen.
