Defacing the Humble Stone

Defacing the Humble Stone is an unmarked quest in Fallout: New Vegas.

Detailed walkthrough
When visiting the War Memorial at Boulder City, the Courier will meet an NCR soldier, Private Kowalski. He is paying respects to his brother, who died during the First Battle of Hoover Dam. Keep being disrespectful or apologize (Speech 30).
 * Aim at the Memorial and Kowalski will become angry; shooting at it will make him furious.
 * If the Courier insults him and his brother, the Courier receives negative Karma and makes Kowalski hostile. Killing him this way doesn't make other people hostile.
 * If the Courier doesn't apologize, they receive NCR infamy. If they shoot at the memorial, this makes all people hostile, including Kowalski.

Whether the Courier has completed the quest or not, Kowalski will disappear after a few days.

Bugs
Kowalski may state that the Courier defaces the monument when they did not. The Courier will have to pass the Speech check to avoid NCR infamy and once passed, Kowalski will no longer speak to them.