You said "ever" so let's go back in the day.... Christmas, 1992.
I was living in Washington DC, one of my co-workers was raised in Milwaukee. DCA-ORD was $800 with no Saturday night stay, DCA-MKE was about $250. I had to go to a meeting in ORD.
So I book DCA-ORD on December 10, returning MKE-DCA December 28. And ORD-DCA December 11, returning DCA-MKE December 21.
I fly the DCA-ORD trip and hand the tickets to my coworker, who gets a free trip home for Christmas, and my company saves about $300. You didn't need ID's back then.
But wait, while he is flying on on of his legs, I'm on another flight to somewhere (I forget where). I'm getting miles twice at the same time.
But wait, there's more! UA at that moment has a promotion, fly 4 segments, get a free domestic RT. (Basically a saver award today.) So I also get a free ticket for my friend flying as me.
Those were the days