Don't Tread on the Bear!

Don't Tread on the Bear! is a side quest in Fallout: New Vegas.

Detailed Walkthrough
The exact contents of this quest vary per your actions. It can result in one of five outcomes:
 * The NCR has a robust network of informants. If you continue to work with Mr. House, the NCR will no longer accept your help.
 * The NCR has a robust network of informants. If you continue to work with Yes Man, the NCR will no longer accept your help.
 * Little escapes the eyes of NCR Rangers. If you continue to work with Caesar's Legion, the NCR will brand you a terrorist.
 * The NCR has declared you a terrorist. Wherever the NCR has jurisdiction, you will be shot on sight.
 * The NCR knows that you are working against its interests and will no longer accept your help.

The first three actions are, obviously, the result of aiding an opposing faction. The fourth is a result of gaining general NCR infamy, usually through killing their troops. In the case of opposing faction quests, it will first activate as a warning upon the completion of a certain quest in the series. Completing another quest will fail the mission. You will no longer be able to take on NCR quests once this happens, and any NCR quests active will immediately fail, as well. In the latter two cases, the entire faction will be hostile to you and NCR rangers will be sent to kill you.

Behind the scenes
"Don't Tread on the Bear!" is a reference to the "Gadsden Flag", a historical flag that depicted a coiled rattlesnake over the phrase "Don't Tread on Me."

Bugs

 * This quest will not fail and you may complete NCR quests if you initially trigger it with House or Yes Man's quest line and then proceed with the other one's quest line. There are two separate quests here actually (one for House and one for Yes Man), and one will not trigger if the other one is active. This is probably a bug, but it remains as of v1.2.