Scope lag is a specific type of lag in Battlefield:Bad Company where the reticule of the scope first appears to be misaligned with the scope itself. Then when you move the left thumb-stick, the reticule moves while the entire black screen stays. if the lag is bad enough, you can move away from the black screen and even see your hands.It's really annoying.
Scope lag is a reminder that it's just a game because scope lag doesn't happen in real life-hopefully.