Incidents demonstrating the theme of the foolishness of mankind are seen all throughout the play. Below is an example of one incident.
We actually see the first incident that supports the theme of the foolishness of man in the very first scene. Egeus is commanding his daughter to marry Demetrius and going before Duke Theseus to petition her punishment if she continues to refuse, even though it is pointed out that Egeus really has no valid reason to prefer Demetrius over Lysander. In fact, it is pointed out that Egeus's preference is very irrational. Lysander points out the irrationality of the preference by asserting that he is equal in social status and wealth to Demetrius, if not even wealthier, as we see in his lines:
I am, my lord, as well derived as he,
As well possess'd ...
My fortunes every way as fairly rank'd,
If not with vantage. (I.i.101-104)
In addition, Lysander points out the irrationality of Egeus's preference by pointing out Demetrius's questionable character and faithfulness. Lysander points out that Demetrius was known to have seduced Helena who is now deeply in love with him. What is more, Theseus supports Lysander's accusation by stating, "I must confess that I have heard so much" (113). Hence, we see through these reasons that Egeus's preference for Demetrius is unfounded and his desire to have his daughter punished irrational, proving the foolishness of mankind.
Puck say this in the play " Lord, what fools these mortals be!"
Some examples of foolishness are Bottom, who mixes up his words. He is also made a fool of, when he is given the head of an ass.
Demetrius shuns the woman who loves him (Hermia) and chases the woman who does not love him (Hermia).
Hermia disobeys her father and the Duke even though she knows that the result could lead to her death / life in a nunnery.
The foolish behaviour of the lovers is due to Puck and Oberon's interferance (magic eye drops) and not of their own making. It is easy for Puck to blame them for the foolishness but he fails to see he is the cause.
Duke Theseus also implies foolishness/madness when he compares lovers to madmen. He says, "lovers, like madmen and poets, are fantasists, "of imagination all compact " (Act 5, scene 1, 8).