Question

Which event in Shakespeare's Twelfth Night triggers the series of incidents related to mistaken identity?

Answer 1

This is when Viola is thought to be Sebastian or vice versa.  This causes confusion as several other characters take on disguises that further complicate the plot in the play.  This is what happens due to the disguises used by the characters here.

Answer 2

B. Viola's decision to dress as a man and work in Orsino's court