What role does misunderstanding play in A Midsummer Night's Dream?

In Shakespeare's A Midsummer Night's Dream, a misunderstanding sets in motion a series of events which causes the four young lovers—Hermia, Lysander, Demetrius, and Helena—to be involved in the mistaken identities and the complications that ensue between and amongst them in the forest.
Oberon misunderstands the situation with the young lovers. He knows about Helena's pursuit of Demetrius, but he doesn't know about Lysander's love for Hermia and her love for him.
Oberon sends Puck into the world to find the "love-in-idleness" flower, the juice of which causes a person to fall in love with the first person they see when they wake up, so that he can use it on Titania.

OBERON: Fetch me that flower, the herb I show'd thee once.The juice of it on sleeping eye-lids laidWill make or man or woman madly doteUpon the next live creature that it sees . . .
PUCK: I'll put a girdle round about the earthIn forty minutes. [2.1.172-175, 178-179]

Puck finds the flower in far less than forty minutes. Oberon takes some of the flowers to use on Titania, which ultimately causes her to fall in love with Bottom, who has an ass's head.
Then Oberon decides to cause Demetrius to fall in love with Helena, and he sends Puck into the forest with the rest of the flowers to put the juice into Demetrius' eyes, and he described Demetrius as wearing Athenian clothing:

OBERON: . . . Take thou some of it, and seek through this grove:A sweet Athenian lady is in loveWith a disdainful youth; anoint his eyes;But do it when the next thing he espiesMay be the lady. Thou shalt know the manBy the Athenian garments he hath on. [2.1.264-269]

So far, so good. But Puck mistakes Lysander for Demetrius:

PUCK: . . . Who is here?Weeds of Athens he doth wear:This is he, my master said . . . [2.2.70-72]

and puts the love potion into Lysander's eyes.
When Oberon discovers Puck's mistake, he sends Puck to put the flower juice into Demetrius' eyes.
When Lysander wakes up and sees Helena, and Demetrius wakes up and sees Helena, they both fall madly in love her, and the mistaken identities and merry mix-ups begin.

PUCK: . . . Lord, what fools these mortals be! [3.2.116]

Puck has some confusion as to whose eyes Oberon wants him to place the love potion drops in. Oberon has observed Demetrius's rejection of Helena's love and wants the love potion put in Demetrius's eyes, so that he will wake up and fall in love with Helena. By mistake, Puck puts the drops in Lysander's eyes. Then, when he realizes his error, he also puts the drops in Demetrius's eyes.
This love potion confusion leads to both Lysander and Demetrius falling hopelessly in love with Helena. Helena, however, suspects this to be a cruel joke the two have concocted to make fun of her, because she knows (or thinks she knows) they are both in love with Hermia. She yells at them to stop tormenting her, but they persist. Hermia gets angry at Helena, thinking she has "stolen" Lysander from her. Lysander and Demetrius are ready to have a possibly lethal sword fight over Helena, Helena thinks it's all a nasty joke, and Hermia is ready to rip Helena apart due to this effort to spread love around.
This error leads to a comic scene with dark undertones and also highlights one of the play's themes: that love is irrational.