On "The Defector" mission, at the part where you go inside the building with Bowman, Reznov, and Woods, (the one where the man is crying over his dead friend), go upstairs. There will be two VC in front of you and three more behind the window next to them. Aim precisely so that you can kill all three of them in one shot. This is a checkpoint. If you miss, kill yourself and try again.