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. Most notable of these encounters being the three alien surveyors past the 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 you have bad reputation with the Powder Gangers, a random Powder Ganger will appear and initiate a conversation, then turn hostile and attack you.

If you have 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 3 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 your first Star Bottle Cap, a roaming trader named Malcolm Holmes will start to follow you from an unknown location. Once he reaches the player, he will converse with them about the caps. He will also start the quest The Legend of the Star if the player asks him to go into more detail concerning the caps. The player can learn more information from Holmes or leave him be. If the player chooses to attack Malcolm Holmes, he will retaliate, but will die quite easily. Looting the body, the player will find several items including four to seven Sunset Star Bottle Caps. You can pickpocket the Sunset Star Bottle Caps off of him as an alternative.

NCR Rangers
While traveling the Mojave Wasteland with a negative reputation with the NCR, 2-3 NCR Rangers will appear, telling you to change your reputation with them for the better within three days, or else they'll attack you "without the pleasant conversation." If you don't listen to him/her, they'll come back and shoot on sight. Most of the time there will be an NCR ranger with an anti-materiel rifle on them and the NCR Ranger combat armor. If the player is a high enough level, one of the rangers will be an NCR Veteran Ranger.

Legion assassins
If you have a bad rep with Caesar's Legion while traveling the wasteland, you will be approached by two Legionary assassins who will initiate conversation and then become hostile. Unlike the NCR Rangers, they will not give you any time to improve your reputation with Caesar and attempt to murder you on the spot.

Kings Gang Member
If you have a good reputation with Freeside, you may be approached by a Kings gang member when walking around Freeside. They will praise your efforts and then give you 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. They appear to be running from a mole rat that is chasing them.

Emergency Radio
If you have a positive reputation with the NCR, a ranger may run up to you and give you a NCR emergency radio which can give you 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 they defeat the Legion, they will reach Novac and from then on make regular stops at the town.