Fallout: New Vegas Scripted Encounters

Fallout: New Vegas random encounters
The encounters in New Vegas are different from previous Fallout games, the handful of encounters being scripted encounters, and not a random encounter from a pool being initiated in predetermined spots in the wasteland. Several of these random encounters can be replaced by the Wild Wasteland trait. Examples of these encounters include the three alien surveyors past Brooks tumbleweed ranch near a hovering alien ship, or the three old ladies with rolling pins in Freeside by Cerulean Robotics where the sexbot Fisto is located.

Powder Ganger Event
Potential to be encountered anywhere, as long as one has a bad reputation with the Powder Gangers, a random Powder Ganger will appear and initiate a conversation, then turn hostile and attack.

If one has cleared out the NCR Correctional Facility, three Powder Gangers show up south of Primm, over the hill from the water tower, north of the Nevada Highway Patrol station. They are always hostile, though sometimes will initiate a conversation and say that they are getting revenge. They respawn every three game days. This event can also occur with three Powder Gangers even if the NCR Correctional Facility isn't cleared out.

Sunset Sarsaparilla Star Bottle Cap
After obtaining one's first Sunset Sarsaparilla star bottle cap, a roaming trader named Malcolm Holmes will start to follow the Courier from an unknown location. Once he reaches them, he will converse with them about the caps. He will also start the quest The Legend of the Star if the player character asks him to go into more detail concerning the caps. The Courier can learn more information from Holmes or leave him be. If they choose to attack Malcolm Holmes, he will retaliate. Looting the body, the player character will find several items including four to seven star bottle caps. The caps can alternatively be pickpocketed from him.

NCR Rangers
While traveling the Mojave Wasteland with a negative reputation with the NCR, two or three NCR rangers will appear, telling the Courier to change your reputation with them for the better within three days, or else they'll attack "without the pleasant conversation." After this period of time, they'll come back and shoot on sight. Most of the time there will be an NCR ranger with an anti-materiel rifle wearing NCR Ranger combat armor. If the player character is a high enough level, one of the rangers will be an NCR Veteran Ranger.

Legion assassins
If one has a bad reputation with Caesar's Legion while traveling the wasteland, they will be approached by a group of four legionary assassins who will initiate conversation, exclaiming that, "Caesar has marked you for death, and the Legion obeys! Ready yourself for battle!" and then becomes hostile. On subsequent attacks, they will not engage in conversation first. Unlike the NCR rangers, they will not allow any time to improve one's reputation with Caesar and will attack immediately.

Kings Gang Member
If one has a good reputation with Freeside, the Courier may be approached by a Kings gang member when entering one of the Freeside gates. They will praise one's efforts and then provide a random item, such as food, caps, ammo or stimpaks.

Jackal Gang Members
Four Jackal gang members will start running in a straight line and will not interact with the player character. They appear to be running from a mole rat that is chasing them.

Emergency Radio
If one has a positive reputation with the NCR, a ranger may approach the Courier and give them an NCR emergency radio which allows for the repeatable quest Emergency Radio.

Caesar's Favor
If you have a positive reputation with Caesar's Legion, a disguised frumentarius may run up to you and give you the quest Caesar's Favor.

Caesar's Hire
If you have a mixed reputation with the Legion, a disguised frumentarius may run up to you and give you the quest Caesar's Hire.

Overpass Merchants
North-east of the Wolfhorn ranch, two traveling merchants, plus a pack brahmin and two mercenaries, will randomly spawn on an overpass bridge should the player approach it from the west. They will try to reach Novac as the Legion ambushes them; if the merchants and mercenaries defeat the Legion, they will reach Novac and from then on make regular stops at the town.