Sometimes casual and seemingly innocuous events in Shakespeare's Romeo and Juliet lead to tragedy. Perhaps we can say that this is one form of tragedy.
For example, in Shakespeare's Macbeth, Macbeth's life ends in tragedy because he traffics with witches and loses his soul in order to be King. However, in Romeo and Juliet, it is a series of "unfortunate events" which lead to the tragic end of the lovers.
When Romeo and Juliet meet at the Capulets' party, it is a seemingly innocent moment: in fact, they fall in love. For Romeo, it is love at first sight:
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiop's ear—
Beauty too rich for use, for earth too dear! (I.v.46-49)
However, Tybalt recognizes Romeo's voice—the son of the Capulets' archenemies, the Montagues. Tybalt is insulted and becomes angry, even though Lord Capulet is not bothered by Romeo's attendance.
Content thee, gentle coz, let him alone.
He bears him like a portly gentleman,
And, to say truth, Verona brags of him
To be a virtuous and well-govern'd youth. (68-71)
The law of hospitality demands that Capulet show Romeo every courtesy, even if the Montague has crashed his party. Tybalt refuses to listen to his uncle's advice. In that the party is Lord Capulet's and not Tybalt's, Capulet eventually tells Tybalt to stand down or leave...that if Capulet is comfortable with Romeo's presence, who is Tybalt to take offense? And so Tybalt retreats. However he promises that what seems like a peaceful response on his part will turn to "bitter acid."
Patience perhaps meeting with a willful temper
Makes my flesh tremble in their differences.
I will withdraw, but this intrusion shall,
Now seeming sweet, convert to bitter acid. (94-97)
This sentiment is an example of foreshadowing, of the trouble that will arise between Romeo and Tybalt. For Tybalt will go looking for Romeo and get into a dispute with Mercutio. Romeo, newly (and secretly) married to Juliet will approach Tybalt in peace, but Tybalt will kill Mercutio; in a sudden act of revenge, Romeo will kill Tybalt, Juliet's cousin.
In punishment, rather than sentencing Romeo to death, Prince Escalus will banish him from Verona. Romeo will flee to Mantua. Juliet's parents will try to force Juliet to marry Paris (who she does not love...while she is also married to Romeo), and Juliet will drink a potion from Friar Lawrence to bring on the semblance of death from suicide. Living in Mantua, Romeo will be unaware of this plan—as will all but Friar Lawrence—and Romeo will be mistakenly told his wife is dead.
Romeo then returns to Verona in secret, kills Paris in self-defense (for Paris blames Romeo for Juliet's death), and then Romeo will kill himself. Juliet will wake, and seeing her husband dead, will take her own life. Even Romeo's mother will die from the heartache of losing her son in exile.
A most casual event—Romeo's attendance at a party—leads to the tragic end of the lovers, as well as Mercutio, Tybalt, Paris and Lady Montague.