Fallout: New Vegas Scripted Encounters

Fallout New Vegas Random Encounters
Several of the Random encounters can be replaced by the Wild Wasteland perk. Most notable of these encounters being the three alien surveyors past the ranch near a hovering alien ship, or the three old ladies in Primm with rolling pins.

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 attack you.

Sunset Sarsaparilla Star Bottle Cap
After obtaining your first Star Bottle Cap (confirmed) a roaming trader named Malcolm Holmes will start to follow you from a location unknown, once he reaches the player he will converse with the player about 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. The Sunset Star Caps can be pickpocketted off of him.

NCR Rangers
While traveling the Mojave Wasteland with a negative rep with the NCR, 2-3 NCR Rangers will appear, telling you to change your reputation with them from bad to good 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'll be an Anti-materiel rifle on them and the NCR Ranger combat armor.

Legion assassins
If you have a bad rep with Caesar's Legion whilst traveling the wasteland you will be aproached by two legion assasins 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.