What mistake does Puck make when placing the "love potion" on the Athenian youth's eyes, as instructed by Oberon?
In Shakespeare's A Midsummer Night's Dream, Oberon sees how much Helena loves Demetrius. Oberon instructs Puck (his fairy henchman) to seek out a flower in a place they once visited. While Puck could not see, Oberon watched Cupid fire an arrow, with a love potion on its tip, which missed its mark and fell to earth. Oberon knows where that potion lies, and he tells Puck to retrieve that flower.
That very time I saw, but thou couldst not,
Flying between the cold moon and the earth
Cupid all arm'd; a certain aim he took
At a fair vestal throned by the west,
And loosed his love-shaft smartly from his bow,
As it should pierce a hundred thousand hearts;
But I might see young Cupid's fiery shaft
Quench'd in the chaste beams of the watery moon;
And the imperial votaress passed on,
In maiden meditation, fancy-free.
Yet mark'd I where the bolt of Cupid fell.
It fell upon a little western flower,
Before milk-white, now purple with love's wound,
And maidens call it love-in-idleness.
Fetch me that flower, the herb I show'd thee once.
The juice of it on sleeping eye-lids laid
Will make or man or woman madly dote
Upon the next live creature that it sees. (II.i.158-175)
When Puck returns, Oberon tells the fairy to take some of the potion and find the "disdainful" Athenian youth—who he will recognize by his clothes; he must place the potion on his eyes so that the first thing he sees when he wakes is Helena, but he must be careful with how much he uses—Oberon wants the "Athenian youth" to love Helena more than she loves him. And so Puck goes off to do his lord's bidding.
Take thou some of it, and seek through this grove:
A sweet Athenian lady is in love
With a disdainful youth; anoint his eyes;
But do it when the next thing he espies
May be the lady. Thou shalt know the man
By the Athenian garments he hath on.
Effect it with some care, that he may prove
More fond on her than she upon her love. (II.i.264-271)
The audience discovers the problem in Act Three, scene two; the difficulty with the plan is that while Puck does find an Athenian youth, as Oberon instructed, it was Lysander, not Demetrius, whose eyes were touched with the magic potion. When Lysander awakes, Helena is nearby and he sees her first, and so instead of loving Hermia, Lysander falls in love with Helena. Oberon is not happy that true love between Lysander and his sweetheart has been disrupted. He tells Puck to find Helena and bring her there, if necessary by using an illusion. He plans to put the "love juice" on Demetrius' eyes (who Oberon has made to fall asleep).
When Helena arrives, Demetrius wakes and falls instantly in love with her. The humor of the play now focuses on two men madly in love with Helena, where before there were none. They are foolish in their pursuit of her, but Helena believes they are having fun at her expense and berates them both.
It will be left to Oberon to arrange for a "remedy" to this "comedy of errors."