Agree with Jon Toner.

The airline's "product" are their service classes and they are priced accordingly.

IMHO, this is no different than stealing. Paying for one product and then taking another higher priced one that you never purchased or were given.

I've only seen it happen once...when a UA Flight Attendant noticed one more PAX in First than her on-board manifest indicated, and questioned the gentleman. He told her he had upgraded at the last minute, and he couldn't find his new ticket stub. She thanked him and walked away, but she must have been suspicious. A few minutes later she returned to him with a reply communication that she had requested the Captain to send. Seems there was no record of an upgrade for this "gentleman." He just turned to me and grumbled, "Well, I tried." and headed back to Coach without even an apoligy to the FA to whom he had lied to.