Beware the Wrath of Caesar!

 is a unmarked quest in Fallout: New Vegas.

Quick walkthrough
The Legion has spies everywhere. If you continue to work against its interests, you will be marked an enemy.

Detailed walkthrough
Activating the quest can be done through several means. Completing the main story line quests of any other faction (the NCR, Mr. House or Yes Man) can activate it, as can gaining infamy with the Legion. In the former case, the quest will activate as a warning. Completing one more mission will cause it to fail. If triggered by Legion infamy, it will activate and fail on the spot. It will only activate after you have been cleared of your Legion infamy (if applicable) following the completion of Ring-a-Ding-Ding!.

Once failed, the Courier's reputation with Caesar's Legion will immediately turn negative, which in turn may cause the appearance of Legionary assassins by triggering Caesar's Foe. The Courier is also proclaimed "in damnatio memoriae" (condemned in memory).