One time the team was fighting a young adult black dragon named Zetagon. They had found the plans for an airboat and had the Queen commision it built. Then they headed out north flying above the amassed kingdom's army to face Zetagon and his hordes.
Did I mention his hordes. I need to. Seemed Zetagon hung out with a rough crowd that included wyverns and hobgoblin commandos who rode the wyverns and dropped down onto the deck of the airboat. One of the characters sharpshot a wyvern out of the sky, dropping the beast and its crew of hobgobbos to the ground 200' below.
This angered three of the wyverns who then landed next to that character. Most of the crew at this point had succombed to Zetagon's breath attack and not many people (PC or otherwise) were left onboard. On this, the top, deck there were just three good guys and four bad guys (a hobbo was fighting somebodies hireling at the very stern). More were fighting down below decks, but that has no bearing on this story of crazy die rolls.
Okay, the three wyverns attack this sharpshooter PC with their scorpion tails. Two hits and his fort saves were 1s. Both of them. Back to back ones.
Then the sorceress that was clear to the aft shot a mystic fire bolt (scorching ray?) at the third wyvern (who hadn't attacked just yet). She rolls a one. That's three ones piling onto this guy's fate.
Then the third and final wyvern takes it's turn and hits again with that tail. A hit. Fort save ... one! Another jumping one! Four ones and this guys Con hit zero. (negative actually)
To top it all off, it looked like the good guys were going to lose. The final rounds of battle saw the paladin get grappled by Zetagon who dropped him overboard. Zetagon only had five hit points left but nobody else (a bard, the out-of-spells sorceress, and a low level NPC sailor) really had a chance. So Zetagon figures he's got it made with his tough scales and no incoming spells and waits for his breath to catch back up.
The paladin is one round away from hitting the ground. He draws his composite longbow and takes a shot. I forgot what he needed to hit, but he rolled a ... twenty! And another twenty to confirm.
The paladin goes splat but Zetagon loses all his hit points from a lucky-shot arrow crit and drops ,too. Right on top of the paladin!
Moral: Ones may doom your character buy a twenty doesn't necessarily save them either.