No rest for the wary is an, achievement/Trophy in Call of Duty: Modern Warfare 2, it requiers you to Knife an enemy without him ever knowing you were there in Single Player or Special Ops
Guide[]
The easiest place to get this is in the Cliffhanger mission in the campaign. When you are told to go plant the C4 by yourself, you will probably come across an enemy that is in a house sleeping. Just walk up behind him and knife him
Video Guide[]
Modern Warfare 2 - No Rest For The Wary Achievement