A/G Cannon verse: they have just gotten married and Merlin keeps walking in on them without knocking, especially before things get steamy between them. They decide to teach him a lesson he will never forget.
The first time it happened she was sitting in Arthur's lap at the long table. The king's face was in the queen's cleavage, and her head was thrown back. She was laughing.
Merlin had heard the laughter, but he assumed it was safe to enter. Surely people don't laugh while they're…
He was proved wrong.
The second time, he was treated to the sight of Guinevere's knees and shapely calves. She was seated on the table this time, Arthur standing between her spread knees, kissing her rather deeply, his one hand roaming up under her skirts.
People eat at that table. Merlin knows that the next time he sits at that table, he's going to be thinking of… that.
The third time was even more horrifying. He walked in to find Arthur seated at the table, alone. His eyes were closed and he was moaning, but his hands were in plain sight.
At first Merlin thought that the king had been poisoned.
"Arthur, are you all—"
"MERLIN!"
Thump.
Then, from beneath the table: "Ow."
Merlin doesn't even remembering leaving that time. He'd never moved so fast in his life.
The final time, he opened the door to find the king lying on his back, chained to the table, wearing nearly nothing. Guinevere is standing over him, standing on the table, wearing only her tall riding boots and one of Arthur's tunics. She has a riding crop from the stables in her hand.
She turns her head coolly towards the apoplectic Merlin and simply says, "You're next."
As Merlin bolts from the room, he hears Arthur's voice behind him.
"Well, that should teach him to knock."
Merlin closes the door and leans back against it, breathing heavily, pinching his eyes closed, trying to will the images from his mind. Especially the ones involving the queen's supple brown thighs.
Faintly, on the other side of the door, Arthur speaks again.
"Are you going to finish what you started, or what?"