This is a great question. There are several illusions in William Shakespeare's A Midsummer Night's Dream.
First, Titania is put under a spell by Puck. When she first sees the enchanted Nick Bottom, with a donkey's head, she falls in love, believing him to be the most handsome creature she has ever seen.
In terms of the four Athenian lovers, a spell is also placed on Lysander by mistake. The potion, as with Titania, makes one fall in love with the first person he or she sees. Puck mistakenly takes Lysander for Demetrius. Lysander, who is already in love with Hermia, sees Helena first, and falls in love with her. Demetrius is also enchanted, as was Oberon's intent, and sees Helena (also as Oberon had planned) and falls in love with Helena.
One of the singularly most entertaining parts of the play is the comedy revolving around the two men who now love Helena, when before, no one had loved Helena. Hermia, is—of course—puzzled by the change in Lysander, and then devastated. Soon she also becomes vengeful, trying to punish Helena for what she believes was a purposeful plan to lure Lysander away. Helena believes that both men are using her for their sport, to make fun of her, and she is disgusted with both of them.
By the end of the play, the illusion of love visited upon Titania and Lysander is removed by Puck (at Oberon's direction) using an "antidote," while it is left on Demetrius, in the name of love.