For what occurrence might Oberon's magical herb be a metaphor in A Midsummer Night's Dream?
The term "falling in love" itself implies Shakespeare's motif of imbalance in his romantic comedy, A Midsummer Night's Dream. Thus, the magical herb which Oberon in Act II, Scene 1 about which Oberon gives Puck orders,
Fetch me that flower; the herb I shew'd thee once:
The juice of it on sleeping eye-lids laid
Will make a man or woman madly dote
Upon the next live creature that it sees.
Certainly, this metaphor of love out of balance is exhibited by Romeo in another of Shakespeare's plays as, without any herb, the young, impulsive Romeo falls madly in love with the daughter of his mortal enemy, the fair Juliet. Thereafter they are "star-crossed lovers," out of balance with the universe.
Likewise, when Puck disseminates this flower's magical powers, it throws the young humans into an asymmetrical love as of the four, one young woman has two men in love with her while another woman has no lover. This imbalance must be set aright for harmony to be achieved; therefore, the play concludes with the symmetry of couples restored, and the madness of love's imbalance ends.