The first Call of Duty: Black Ops had a killstreak which allowed players to call up a SAM Turret, which automatically locked onto aircraft killstreaks to fire missiles at them and destroy them.
With the exception of the Blackbird, the turret was capable of taking down any airborne killstreaks, but it was equipped with a limited number of missiles and completely defenseless against ground targets. It was especially vulnerable to knife attacks, which allowed players to kill it a lot faster than they otherwise would with a ranged weapon.
In a recent tweet by Treyarch’s game design director David Vonderhaar, Treyarch is aware of this issue and intends to make it so that the turrets are invulnerable to knifing attacks in the upcoming Black Ops 2, where they’ll be available as scorestreaks for players to summon.
According to Vonderhaar, the ability to knife these turrets to death was ruining the balance of the game as being so easily destroyed decreased their value.