Don't Tread on the Bear!

Don't Tread on the Bear! is a side quest

Detailed walkthrough
The exact contents of this quest vary per actions. It can result in one of five outcomes:
 * The NCR has a robust network of informants. Continued work with Mr. House makes the NCR refuse the Courier's help.
 * The NCR has a robust network of informants. Same as above, but when working with Yes Man.
 * Little escapes the eyes of NCR Rangers. Continued work with Caesar's Legion gives the NCR's brand of a terrorist.
 * The NCR has declared the Courier a terrorist. Wherever the NCR has jurisdiction, be prepared to be shot on sight.
 * The NCR knows that the Courier is working against its interests and will no longer accept the Courier's help.

The first three actions are the result of aiding an opposing faction. The fourth is a result of continuing to work with Caesar's Legion, gaining general NCR infamy, usually through killing their troops, or launching the missile at Long 15. 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. The Courier 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 fourth action, the entire faction will become enemies with the Courier, giving a preparation to be attacked. The fifth action will result in the Courier no longer being able to further advance through the NCR quest line as a result of continuing to work with Mr. House or Yes Man. In either case, the NCR-based ending is no longer possible to achieve.

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."