Defacing the Humble Stone

 is an unmarked quest in Fallout: New Vegas.

Walkthrough
When visiting the War Memorial at Boulder City, you will meet an NCR soldier, Private Kowalski. He is paying respects to his brother, who died during the First Battle of Hoover Dam.

Aim at the Memorial and Kowalski will become angry, shooting at it will make him furious. Keep being disrespectful or apologize (Speech 30).

Bugs

 * Sometimes Kowalski will not be present, regardless of how quickly the player makes it to Boulder City. The reason for this is unclear.