Puck mistakes Lysander for Demetrius due to a misunderstanding with the orders he receives from Oberon. In act 1, scene 2, Puck is sent to retrieve “Love in Idleness,” the purple flower which can work as a magic love potion. While Puck is gone, Oberon witnesses a scene between Demetrius and Helena. He feels sorry for Helena, and when Puck returns, along with ordering him to administer the love potion to his queen Titania, he tells Puck:

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 (1.2).

Unfortunately, Oberon’s instructions were not clear enough. He intends for Puck to give the potion to Demetrius, but when Puck comes across Hermia and Lysander in the woods, he naturally assumes that this man in the “Athenian garments” is the one the potion is meant for. Because Lysander and Hermia are asleep, Puck could not have watched them interact with each other or check if they were the correct couple, even if he wanted to.