The javelin glitch in Modern Warfare 2 is currently being exploited for great and evil purposes. Watch this video to learn how to take advantage of this glitch, for both the XBox 360 and the PS3. Fair warning, though: doing this glitch could get you banned by the powers that be at XBox Live.
This glitch is good and really simple to do. All you have to do is have a class with a javelin and a semtex grenade. First get your javelin out and then cook your grenade. While you're holding your grenade (don't throw it), press Y, triangle on PS3) to switch back to your javelin. Then, you die and your javelin will go off and kill any enemy around you. This glitch is really good against campers or people boosting to get a nuke or any other type of killstreak.