He may claim to be "humbled" by his past behavior, but Chris Brown is still losing his infamous temper. Just nine months after being sentenced for assaulting then-girlfriend Rihanna, the 20-year-old singer became enraged when he was denied entrance to Eve Nightclub in Las Vegas for being underage on May 1. "He went ballistic on the bouncers and kept repeating, 'Do you even know who I am?'" an onlooker tells In Touch of the hotheaded star, who performed the national anthem earlier in the evening at the Floyd Mayweather Jr. and Shane Mosley fight. His tantrum wasn't the only controversial part of the night - given his February 2009 attack on songbird Rihanna, many found Chris' decision to sing at a boxing match to be in poor taste. The "Forever" crooner hardly took the criticism in stride. "Im human. how many of u r perfect????????," he fumed on his official Twitter page the next day.