in Act I Scene 5, the countess falls in love with Viola, who is disguised as a man. She admires Viola/Cesario's speech, looks, and general attitude and style.

She says:

Methinks I feel this youth's perfections
With an invisible and subtle stealth
To creep in at mine eyes. Well, let it be.--

and then, at the very end of the act, she goes on to say:

I do I know not what: and fear to find
Mine eye too great a flatterer for my mind.
Fate, show thy force. Ourselves we do not
owe:

Which is her way of saying she thinks she is falling in love, but is unsure exactly.