The use of thrown combat knives by elite soldiers to take out sentries is mostly Hollywood myth. However, there is one exception. Russian special forces troops, since Cold War days, have received extensive training on how to kill enemies by throwing knives. Observers have reported that the soldiers actually become quite expert at accurately throwing them over considerable distances. Spetsnaz troops arent issued a knife designed specifically for throwing, but are issued the Army Knife produced at the IZHMASH weapons factory. The knife weighs about 2 pounds and has a blade length of 6 inches and an overall length of 13 inches. The knife is double-edged, with one edge being completely serrated, making it an excellent weapon for hand-to-hand fighting. The handle is hollow and comes with a small survival kit.
Whether this training has actually been put to use on the battlefield is a matter for debate, but there are several reasons why elite troops do not usually throw their knives. First, knives provide troops with an invaluable survival/everyday multipurpose tool with which they open cans, cut rope, pry open ammunition crates, and do a multitude of other tasks. To throw one at an enemy and miss is essentially throwing away a very useful, and possibly expensive, tool.
Secondly, although edged weapon training is an important part of many special forces schools, troops are generally not taught to throw them because there is too much of a likelihood that they will miss. If that happens, the soldier has not only thrown away a perfectly good weapon, but will have to resort to his bare hands to engage the enemy, which should always be a last resort. When silent removal of sentries is necessary, there are many different ways to accomplish that task that are better. Garrotes, neck manipulation, and controlled use of an edged weapon all ensure more success with less risk of being caught. Of course, the amount of time spent on practicing knife throwing is dwarfed by the time spent on the firing range with their primary weapons. Elite troops try to avoid engaging the enemy with non-projectile weapons as much as possible.
Russian/Soviet troops have always excelled at hand to hand combat training, and a thrown knife is probably very deadly in the hands of a trained expert, but most elite troops choose to stay on the safe side and dispose of enemies with their knives well inside their grasp.