Oberon instructs Puck, his helper, to find a magic love potion and to give it to Lysander so as to make him fall in love with Hermia. However, Puck makes a mistake and because of it, both Lysander and Demetrius fall in love with Helena, which is not something Oberon wanted to happen.
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Initially Duncan is the king in Macbeth, however Macbeth later kills him and becomes king himself.