Standing around like a bleeding idiot
When you shoot someone in the head with the Sniper's Rifle causing critical damage and killing most classes with one shot. The classes that can survive a not fully charged Headshot normally have a medics help or one of the three classes with most health, Heavy, Demoman, and the Soldier but no class can survive a fully charged headshot with or without the medics help. It causes 450 damage when fully charged with a headshot.