What does Oberon realize when he sees Hermia and Demetrius together in act 3, scene 2?

William Shakespeare’s A Midsummer Night’s Dream is a play about the difficulties of romantic love. These difficulties are made worse through the interference of the king of the fairies, Oberon, and his servant Puck. In act 3, scene 2, Oberon realizes that his servant Puck has used the love potion on the wrong person. Oberon wanted Puck to place the love potion in Demetrius’s eyes to make him fall in love with Helena. Instead, Puck, through mischief or mistake, places the potion in Lysander’s eyes, causing him to turn away from his true love, Hermia, and chase Helena. When Oberon sees Hermia and Demetrius arguing, he realizes his servant’s mistake.

“What hast thou done? thou hast mistaken quite And laid the love-juice on some true-love's sight: Of thy misprision must perforce ensue Some true love turn'd and not a false turn'd true.”

Once Oberon realizes Puck’s mistake, Oberon orders him to bring Helena there. Oberon then anoints Demetrius’s eyes to make him fall in love with Helena.
http://shakespeare.mit.edu/midsummer/midsummer.3.2.html