So I'm DMing and the party (two half-elf fighters, a gnome wizard, a tiefling sorcerer, a human ranger, and a dragonborn rogue with levels in bard), is approaching their final boss battle for the campaign- which just so happens to be an adult green dragon.
On their way into the cave however, the rogue falls through a trap that closes behind her. Since the party has no way of getting her back, they decide to go on and end up facing the dragon without her. They give it a good beating but in the end they have to fall back and let it retreat into its cave system. Two of them (one of the fighters and the wizard) were unconscious and the rest were in pretty bad shape.
At this point the rogue who has fought her way through several rounds of goblin combat, defused three traps and is making her way up to the surface, hears the dragon coming down the tunnel she's in. She stealths into an alcove with a natural 20, and the dragon (who failed it's perception checks) walks past without noticing her.
Rogue: If I attack will it be surprised?
Dragon: *fails another perception check*
Rogue: Ok, I'm going to use assassinate with my rapier of sharpness. *rolls 22 to hit*
Rogue: Ok and for my second attack… *rolls natural 20*
Rogue: *rolls damage on a ton of dice*
Party: *start saying goodbye to their friend's character and telling her how she'll be missed*
Rogue: 90 points of damage.
Party: Wait… what?
Me: …how do you wanna do this?
So in the end the rogue managed to behead a (wounded) adult green dragon, with two blows singlehandedly. She then continued to strip it of scales and drag its head back up to the rest of her party. To this day, her character still wears dragon-scale armour that she had made to celebrate her victory.
This is why you don't mess around with the rogue who has a rapier of sharpness, and who has just been through hell.