Remain motionless, as an incoming deer will pick you off!
Remember, you most likely won't see the deer the doesn't walk bye you.

Meaning...
Imagine a deer just about ready to come into your setting, maybe an oak flat or a field edge?
He stands almost motionless while scanning and scent checking for 10 or 15 minutes behind some bushes,
before making his way into your view.
You are unaware of his presence...
Unfortunately you have an itch or a mosquito that your swatting away and he sees you.
He then turns around, never to be seen by you, and walks away.
~ The damaging factor may not be that day's sit, it may be an education for the deer for the rest of the season.
* This also goes hand in hand with scent control.