Merlin does powerful magic, but nobody notices. Screenshots.
Part 01 here. 1. Episode 04, The Poisoned Chalice
In a hall full of people, Merlin exhibits the magical trick known as 'The Vanishing Chalice.'
First, he makes a strong eye contact with the audience.
Then he drinks.
Next, he makes the cup disappear. To bring the attention of the public away from that, he feigns sickness. (The act must be dramatic enough to work.)
To add a bit to the dramatic effect, Merlin falls. Notice how the cup isn't in his hand yet.
When everyone's attention is focused on Merlin's face, worried about his well-being, Merlin conjures the cup again.
('The Vanishing Chalice' was a known seduction technique before Uther's reign, performed mainly by witches. It was the magical equivalent of ladies' giving their garters or dropping a handkerchief for their chosen ones. The practice would have been forgotten, but Merlin was lucky to have found an exact description of the trick in one of Gaius' books. With that he was able to bring the magical trick back to life.
A similar technique is known among the Sidhe, with the difference that the race of Sidhe prefer using insignificant humans instead of a cup. The magical trick differs in another point - whereas the humans like to conjure the cup back at the end of the trick, the Sidhe like to kill the humans. See Sophia's demostration in the episode 'The Gates of Avalon' for more information.)
The trick certainly worked on Arthur, for Merlin has now the total attention of the prince.
As it usually turns out in such cases, Merlin is escorted by Arthur into the privacy of Gaius' chambers.
Unfortunately, Gaius and Gwen are not familiar with this magical custom, and they follow the young couple. Arthur ponders why he didn't chose his own chamber.
With such twist of events, Merlin pretends to be sick, and Arthur brings a flower as confirmation of his feelings toward Merlin.
Since a declaration of love has been made by both parties, the balance of love magic is restored.
Summary: Merlin does some powerful spacial magic in front of a hall full of Camelot courtiers and an expedition from Mercia, but nobody notices because Merlin decides to skillfully fall. Arthur, as usual, is enchanted.
What a prestige!