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.

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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."