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 and sometimes say they are getting revenge. They respawn on a regular basis.

Sunset Sarsaparilla Star Bottle Cap
After obtaining your first Star Bottle Cap, a roaming trader named Malcolm Holmes will start to follow you from a unknown location. Once he reaches the player, he will converse with the player 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.

Inversely, if you have a good rep with the NCR, you may be approached by a soldier who will tell you that the "NCR has your back." He'll give you an NCR Emergency Radio for future use.

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

Kings Gang Member
If you have a good reputation with Freeside, you may sometimes get approached by a Kings gang member when walking around Freeside, telling you to keep up the good work, and handing you a random item, from junk food to ammo to two stimpaks.