When You Know, You Know (смешанная)

"You can take Daisy out to fetch the cloak-"
"How do you know that?"
Arthur closed his eyes, expression riddled with irritation before turning to look at Merlin expectantly.
"Know what?"
“The mare I use. You know I call her Daisy? I never told you that." Merlin explained. Arthur stared for a few moments. He could see the flicker of emotions pass easily over him. Anger, annoyance, exasperation, indignance, and finally acceptance...AKA the Five Stages of Grief (Pendragon Edition).
"You talk to the horse.” He pointed out. Merlin narrowed his eyes.
“I whisper to the horse."
"...am I supposed to not hear it?” Arthur asked. Merlin wondered how likely it would be that he also heard him whispering magical spells if he heard him murmuring to the bloody horse. “Also, only you would choose such a ridiculous name.” He muttered. Merlin swallowed. This wasn't a problem at all, right? Right.
Or
Merlin knows Arthur well. Better than he knew himself apparently. What he hadn't expected was for Arthur to know his servant just as well... This causes problems. (also known as "The Time Merlin Had a Mental Breakdown Over Arthur Being a Normal Human")