Scope Shadow
The dark crescent at the edge of the image that appears when the eye is not centered behind the scope, a reliable cue to fix head position.
Scope shadow is the dark moon-shaped band that creeps in from one side of the view when the eye is off-center or at the wrong distance behind the eyepiece. The direction of the crescent tells you which way you are out of position: shadow on the right means the eye is too far left, and shadow all the way around usually means the eye is too far back. If you press the trigger with shadow in the picture, the rifle is canted or your head is misplaced, and the shot tends to walk in the opposite direction of the dark side.
The cure is a consistent cheek weld that puts the eye in the same spot every time, set at the correct eye relief for that scope. A scope with a generous eyebox is more forgiving and lets you find a clean full sight picture faster, which matters when you are building a position quickly under time. Training yourself to reject any shot that still shows shadow is one of the cheapest accuracy gains available.