Noob Combo

The so-called "Noob Combo" is the combination of the Plasma Pistol and Battle Rifle, Carbine or Magnum. Often one would be walking and hear the splash of the plasma pistol being fired only to turn around and be hit in the face with the giant green blob and then shot in the head with the Battle Rifle, Carbine or Magnum in an instant. This combo was especially rampant on Beaver Creek and Colossus. It was incredibly effective and often turned the tide of games. This tactic was considered so imbalanced that Bungie attempted to remove it from the game by implementing the new Battle Rifle game type which removes all Covenant weapons from the map. This was largely ineffective. They also made a Team ActionSack gametype for it called "combo training" but Bungie managed to shrink this problem a bit when the Halo 3 beta came out, Bungie made the charged shot of the plasma pistol travel much faster causing it to have very little maneuverability, since many players always hold the weapon ready to release the shot, Bungie made the battery power decrease the longer the player held the trigger down.

Whether or not this weapon combination is actually a sign of being a "noob" is up for debate. While it is indeed a powerful and arguably imbalanced combination, it does require some skill to use, and using weapon combinations effectively is not a trait of a new player.

However, even a player who would normally lose to you can kill you at least once with it. The main distinction being that a skilled player will survive the counterattack and go on to get a spree; while an actual noob will easily be taken down once the initial surprise is gone and you know he's using it.

Combinations
Note: the noob combo is executable only with a plasma pistol and a close range headshot capable weapon. The SMG-Plasma Pistol combo is NOT a noob combo for the simple reason that you are unable to do a headshot, although that rule may change due to the fact that many gamers consider duel wielding the SMG or the Pistol with a plasma pistol or plasma rifle is a noob combo. However, this combo is very useful in campaign, especially in the higher difficulties.
 * 1) Battle Rifle and Plasma pistol
 * 2) Carbine and Plasma pistol
 * 3) Dual wielding Magnum and Plasma Pistol

These are the combos that are sometimes considered noob combos too: (Plasma Rifle combos) (Melee combos)
 * 1) Dual Wielding SMG and Plasma Pistol
 * 2) Dual Wielding Spiker and Plasma Pistol
 * 3) Assault Rifle and Plasma Pistol
 * 1) Dual Wielding SMG and Plasma Rifle
 * 2) Dual Wielding Spiker and Plasma Rifle
 * 3) Dual Wielding Magnum and Plasma Rifle
 * 1) Mauler followed by a melee
 * 2) Plasma Pistol followed by a melee
 * 3) Plasma pistol and a single Brute shot grenade

The list could go on (it's literally based on what people consider a noob combo to be). The noob combo was supposed to be a plasma pistol shot followed by a headshot, but as time went by, people began considering fast kills, such as the ones above, to be noob combos.

Blocking It
It is possible to block the combo. The solutions are listed below.

You must be dual wielding and facing your opponent. Wait until the plasma bolt is about to hit you, then quickly press B (melee). If done correctly (properly timed), the plasma bolt will hit the weapon you just threw out. It seems best to drop a Plasma Rifle.

A second method which can block the noob combo is to hit the discharged shot with a grenade. Plasma and Frag Grenades both work. It is argued that a plasma granade is better, because it is a glowing ball of energy that the discharged bolt will home in, while some believe that Frags are better because they fly through the air faster. The Plasma Pistol overcharge may also set off a Frag Grenade that is thrown at it, injuring the person attempting to block the overcharge.

Both the dual wielding and grenade method works and it is about preference on which to use.

Another tactic, which should only be used as a last resort, is to lob a fragmentation grenade in the direction of the blast and then crouch. While the person using the combo is attempting to line up a headshot (slowed down slightly by you crouching, the frag grenade should knock them off balance long enough for your shields to recharge a bit or kill them if you're really lucky). Don't use a plasma grenade because you want an explode-on-impact grenade and it should be noted that against a Battle Rifle, you're screwed anyway.

It may also be blocked by evading. Examples are to jump sideways or move behind a wall.