I wouldn't suggest to do something unlawful or immoral, but I have heard of people who just said they were the person that booked the flight orginially (having the booking locator handy). That way the Agents don't have to worry about doing anything wrong...

When my Dad is having trouble with his ISP, I will call them and tell them I am my Dad. That way there's no trouble with any checks or DP.

When a colleague and I missed a connection, I could only rebook my own flight and had to pass on my mobile to my colleague so that the Agent could talk to him personally (even though I had the record locator and he could see that we both weren't on our flights) - the rules are the rules.