Always look behind what you are shooting at. Tunnel vision and the rush of spotting game can blind you to what's behind the game you are shooting at.
I've had Mushroom hunters sneak up through the woods behind where the Deer normally show. I killed a Doe laying in the tall grass behind a feeding Buck, a double a surely didn't want. I once spotted a TV set in some guys living room a couple of hundred yards down the hill from behind a trophy Buck. Often a bullet goes right through a Deer.