Mysterious Stranger (Fallout 3)

Mysterious Stranger is a perk in Fallout 3.

Effects
If the last target at the end of a V.A.T.S. sequence has 150 health or less, there is a 10% for the Mysterious Stranger to appear. He will then finish off that target using his unique .44 Magnum.

Additional dialogue options
In the add-on Mothership Zeta, when first arriving aboard Zeta, having the Mysterious Stranger perk opens up a new line of dialogue with Somah, with the option "Maybe, if we're lucky, a friend of mine will show up to help us."

Bugs

 * The Stranger will sometimes be positioned unfavorably. This can include being stuck in a wall, being placed in water, being placed near a cliff, etc. This can result in unconsciousness or inability to effectively attack the enemy, or even in some cases to kill the player character if they are between him and his target. Because the Mysterious Stranger never attacks, even if all enemies have died, he will be forever trapped inside the level geometry and the trigger to return the game to normal never happens. The only choice is to load a previous save.
 * During V.A.T.S., it is common to hear the Mysterious Stranger jingle, even if he does not appear. This most likely happens at times when he would have appeared to finish off your enemy, but you killed the enemy instead. He may also have been affected by the bug above in some of these cases.
 * Sometimes when the Mysterious Stranger appears, he does not shoot. He will just stand there, leaving you helpless. Another variant makes the Mysterious Stranger reload his Magnum for the duration of his attack. If it jams, he may not attack at all. This also happens if his target is killed prematurely by an explosion or by you or a companion. V.A.T.S. will remain active for an extended time, leaving you open for attacks by any surrounding enemies.
 * While in the Operation: Anchorage simulation, the Mysterious Stranger will sometimes appear and then lie unconscious. When the V.A.T.S. sequence ends, gameplay will return to normal.
 * At times, the Mysterious Stranger will aid you during V.A.T.S., but there will be a 5-10 second delay between killing the target and returning to normal gameplay. This can pose problems when fighting multiple enemies, as they will continue to attack both you and your companions during the delay. On occasion, this bug may cause normal play to not resume.
 * Sometimes, the game can freeze in the zoomed-in style of V.A.T.S. when the Mysterious Stranger appears, making him stand there and watch.
 * Occasionally, the game may register that the Mysterious Stranger has appeared (you receive a progress update) but he does not, nor does the opponent die. You remain frozen in V.A.T.S. unable to move or die.