I had a situation recently where I am sure a GS stole my seat assignment.
I arrived IAH from the UIO redeye on a R/T paid F ticket, to find I was kicked out of my my perfect 1A seat (on the single aisle side of an RJ) and moved to 1D. I was already checked in and had a BP.
I was gonna let it go since it's only an hour to DFW, but I mentioned it to the UA Club agent. He said it must have been for "special needs". OK, fine.
But THEN I saw the person now in 1A was on the upgrade list. When I pointed it out to him, he suggested I ask the gate agent to look into it, because "something is very odd about this".
The gate agent looked it up and became perplexed, saying this shouldn't happen. She insisted on calling over a a "Team Leader", who looked up both reservations and was also perplexed. She told me he was a GS on an domestic economy ticket, CPU upgraded, and she can't understand how this could have happened.
Then she laughed and said "well, we will just change you back to your original seat!".
I sat down and watched as she called him up to the gate. (obvious who he was because I saw his partial name on the UG list). I couldn't hear from where I was sitting, but he was yelling and waving his arms around (much to my glee).
When they preboarded GS, he RAN onto the plane first. I got on next with the 1Ks and he was sitting in my seat!
I laughed and said "nice try" and pointed my thumb outwards (as in "move your arse!"). The FA came over and he showed his old 1A BP. When I suggested she call the gate agent he got up and moved (huffing, puffing and swearing the entire time).
The Team Leader and the UA Club agent both asked me to please write into 1K Voice about it, but I never got around to it. It was 3 weeks ago -- I wonder if I still should?
I'm sure some FTs will tell me I'm being ridiculous to make such a drama over an hour flight. But I can't help but wonder about next time, when some DYKWIA kicks me out of my desired seat on a 15 hour flight.
So yeah, it happens!