The agent replied, "I am sorry, sir I'll be happy to try to help you, but, I've got to help these folks first, and I'm sure we'll be able to work something out."
The passenger was unimpressed. He asked loudly, so that the passengers behind him could hear, "DO YOU HAVE ANY IDEA WHO I AM?"
Without hesitating, the agent smiled and grabbed her public address microphone, "May I have your attention please, " she began, her voice heard clearly throughout the terminal. "We have a passenger here at Gate 14 WHO DOES NOT KNOW WHO HE IS. If anyone can help him find his identity, please come to Gate 14."
With the folks behind him in line laughing hysterically, the man glared at the United agent, gritted his teeth and swore "F*** You!".  
Without flinching, she smiled and said, "I'm sorry sir, you'll have to get in line for that too."