Disclaimer: I own nothing.
But I unfortunately was born at the wrong end of time, and I have to live backwards from in front, while surrounded by a lot of people living forwards from behind. Some people call it having second sight."
-White, T.H. The Once and Future King
They kiss only once.
Arthur has been acting strangely for weeks, avoiding eye-contact and saying no more than the absolute minimum required. The few times the warlock manages to catch his eyes, the other man's gaze is both bewildered and bereft. No matter how hard Merlin tries, the prince refuses to tell him what is wrong. It is obvious, however, that his issue is with the warlock himself, for Arthur's strange behavior seems reserved for only him. With a heavy heart, the manservant remembers the strangely sad looks Arthur used to give him when he was king and can't help but wonder if those, too, were the result of some grievance he has yet to commit. The idea, that he will somehow truly hurt the man he has come to care so much for, leaves him winded.
Merlin feels no closer to finding the answer when he manages to corner the prince in his chambers. Frustrated by his seeming inability to alter his own future (and thereby change Arthur's past), the warlock lets his current irritation and years of longing take control as he meets the other man's mouth with his.
The kiss is somehow slow and frantic all at once, and Merlin forgets, for one solitary instant, that he is stuck living backwards while everyone else is living forwards. Just this once, he lives in the moment.
Arthur pulls back and, with a voice rendered shaky in anticipation, murmurs, "no, we haven't."
With dawning horror, the warlock realizes that he now knows how his future will go. Merlin, who is so used to living past before present, effect before cause, knows exactly what to say. "We've never kissed before."