The National Anthem
When Borat begins his special appearance at a Virginia rodeo, things actually start out on a pretty good foot. The crowd doesn’t fully grasp parody in the immigrant’s support of the Iraq War, calling it America’s “War of Terror” and calling for President George W. Bush to drink the blood of Iraqi citizens, but things fall apart quickly when he starts singing the national anthem. The audience is with him at the start as he replaces the words of Francis Scott Key with the lyrics from the Kazakhstan national anthem, but he loses them pretty damn fast when he starts talking about all other countries being run by little girls and railing on international potassium exports.
Of course, the bit also has the perfect punchline with the flag-waving man on horseback falling to the ground in the background.