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Twelfth Night,
Act 1, Scene 5, line 294.

Note to Twelfth Night, 1.5.294, "Unless the master were the man"

Apparently Olivia has a sudden suspicion that the "man" (the young gentleman who has just spoken on Orsino's behalf) might be the "master" (Orsino) in disguise. However, it seems that the suspicion goes as quickly as it comes, because Olivia says no more about it. It's hard to see how she could have such an idea in the first place, since she speaks of Orsino as if he is well known to her, and Viola, even in man's apparel, looks nothing at all like Orsino.

Viola considering Orsino
Viola considering Orsino