Heres a list of a few things Ive noticed many noobs do that cause both themselves, and their teammates to die. I don't hate noobs, but when your teammates are working against you it can be frustrating.
1) Doorway Blocking: Noobs often like to stand or crouch in the middle of doorways effectively blocking much better players from exiting the building and killing the enemy. Not only does this stop players from leaving the building, but many times will result in the death of both the noob and the seasoned vet, via collateral damage shots, grenades lobbed into the room, gernade launchers, and various other methods.
2) Care Package Jacking: Noobs loves try to steal care packages, even if it kills them in the process. While this is a more desirable outcome than letting the enemy have the package, there is nothing more painful than, watching a noob waste a wonderful package. Once in the hardcore game mode a guy killed me, stole an AC130 package, and successfully killed three team members(including me a second time), and 0 enemies. Learn how to use kill streaks more effectively so if you do steal one, its not wasted. Oh, and don't team kill.
3) Giving away positions: Many a time have I seen a Noob invade a spot where a sniper was already camping, and starting spraying bullets at far away enemies. This effectively gives away the position of the sniper, usually ending in a grenade related death for both.
4) Shot Blocking: If I had a Nickel for every time a noob walked directly in front of me whilst I was shooting a member of the enemy team and tried to steal the kill, I would have like 10,000 Nickels. While I usually continue shooting, and still manage to kill the guy through the noob, sometimes the enemey will either kill both of us, or get away. Don't stand in front of your teammates.
5)Standing Still: This one does the least harm to the team, and the most harm to the noob. So many times have I seen a noob camping somewhere and just sitting completely still. while they may be hidden, if they do get spotted they will surely die. Also noobs will run out in the open, see an enemy and crouch completely still to get a steady shot, and get annihilated.