in act four
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"Oberon told Puck to put love-in-idleness on Demetrius, instead he put it on Lysander by mistake, then he puts it on Demetrius; both men see Helena on first awaking and scorn Hermia. Oberon tells Puck to put them to sleep when they go to fight a duel and put something on Lysander's eyes to break the charm."
Lunt, Forrest. Shakespeare Explained. New York: Hearst's International Library, 1915. Shakespeare Online. 17 NOV. 2014.