What does Puck have to do at the end of Act 3 in "A Midsummer Night's Dream"?
Becaus Puck accidentally put the love juice of the purple flower into the wrong lover's eyes (lysander), he must go back and change that problem. He must put the love juice into the eyes of Demetrius.
In ActIII sc2. because of Puck mistakenly pouring the love juice in Lysander's eyes and then in order to correct his mistake pouring it in Demetrius' eyes both the men are now in love with Helena. When Hermia joins them Lysander to her shock tells her that he is now in love with Helena. Helena thinks that the three of them-Lysander, Demetrius, and Hermia-have joined together to mock at her. Soon Helena and Hermia begin to quarrel and Helena runs away to save her life. Meanwhile Demetrius and Lysander begin to quarrel and decide to fight a duel to determine who loves Helena the more and Lysander challenges Demetrius:
"Now follow, if thou darest,to try whose right,
Of thine or mine, is most in Helena."
Oberon who observes this misunderstanding realises that Puck is responsible for all this confusion and after chiding him, he orders Puck to cover the entire night and the starry sky with a thick black fog so that Demetrius and Lysander cannot see anything. Oberon then orders Puck to deliberately mimic the voices of Lysander and Demetrius and mislead them in different directions and make them completely exhausted. Oberon tells Puck that once both of them are completely tired and fall asleep he must apply the antidote of the "love in idleness flower" to the eyes of Lysander so that when he wakes up he is cured of his infatuation for Helena so that he will be once again true and faithful to Hermia:
"Then crush this into Lysander's eye
Whose liquour hath this virtuous property
To take from thence all error with his might
And make his eyeballs roll with wonted sight."