In Call of Duty 4: Modern Warfare 2, there's a glitch that you can take advantage of when you have both a javelin and a grenade in your hands. Modern Warfare 2 is pretty much the game of the moment, of the year, of the decade, so exploit this glitch while you still can! Fair warning, though: doing this glitch could get you banned by the powers that be at XBox Live.
You can do this glitch with any launcher and works with no ammo.
For PC and PS3 users, have the javelin (or other launcher) in your hands. Hold down grenade button (Semtex or throwing knife). While still holding grenade, hit your switch weapons button. Now your launcher should be back in your hands and you can let go of the grenade button. You are ready to explode, you can knife, sprint, and jump, but anything else you do cancels it and the process has to be repeated.