Gotta tell this. Read it somewhere a few years ago:
John von Neumann, one of the most incredibly intelligent mathematicians of the twentieth century, called his wife from another city. Was on a flight, switching from one plane to another. He had two questions:
(1) Where was he going?
(2) And why was he going there?

Remember (uh, maybe a different word is more appropriate here): If you forget, it's not necessarily forgetfulness, it's probably genius.

A fella in the row behind me was approached by a guy asking if he'd trade seats with him. They were in exit rows in Y.

He said "hi, you're sitting next to my wife, i'm seating in seat 5b, it's a first class asile seat, want to trade?"

Wanna sit near your relative? That's how you do it... move back don't insist on moving up..

On a flight ARN-LHR on BA a year or so ago I found someone in my seat who had the exact same seat assignment on his BP. Brought over the stewardess who left with our BPs, came back a minute later and told my wife and me that we were flying "in front of the curtain". I guess it was an operational upgrade that should have been caught as we were boarding. In either case I said thanks and enjoyed the hot breakfast.

I was on a flight in June where there were several people who had the same seat numbers on their BPs. Three flyers had checked in online the night before and three who checked in at the airport had the same seat assignments. There was a glitch in the system. There was quite a fuss by several passengers who were very rude to the two who were seating in "their" seats. The two were an older couple flying together and had boarded first. The two who were very rude got "their" seats. The older couple got upgraded to first. The third person who checked in online first also got upgraded to first.
Dawn

On a recent 6 am flight, fully loaded, I encountered two 4 year old sisters, one of whom was sitting in my seat at 21C. I was already a little edgy from the usual lack of overhead bin space and the early-ness of the morning, and I could think of no way to gain my seat without looking like an evil bad-man. Unbeknownst to me, someone else was sitting in one of the sister's seat.

But I knelt down and asked the girl in the most polite voice possible for me at 6 am, "Excuse me, are you sitting in the wrong seat?". And she replied, "No I'm not". I thought Delta made a booking error. Fortunately, she got out her ticket for me to look at, and it was for the window seat at 21A. I repeated the same question to the gentleman in 21A, and he insisted he was in the right seat. After 30 seconds more of haggling, and me looking ever more foolish, he pulls out his ticket to prove me wrong - and it's for 20A, the row in front of us.

Fortunately he realizes his error and moves. No blood is spilled. The rows in our Delta MD-88 are one-off on the three seater side, and unfortunately he looked on the two seater side to figure out which row to sit in. Next time I run into this situation, I'll be sure to look at the other person's ticket first, or get a flight attendant :-)

That's true. I guess it would also depend on who the person is . . . someone mentioned, for example, an older person. I think I might have a harder time in that case insisting on sitting in my assigned seat. Fortunately, though, I've yet to have to deal with the situation.

Fortunately I wasn't involved, but I did see a couple being a problem on a Delta flight recently. The person with the real seat had to get the FA. When she asked to see their boarding passes, they said they gave them to the gate attendent. She then reminded them that they got the stubs back. They then made a big production out of trying to find the passes. I lost track of what was going on with them at this point, but I think they did end up having to move.

Your story leaves me speechless. The person who did that is beyond obnoxious. What did he think you were going to do? I guess he was hoping you would be on of those unassertive people who would just suck it up and sit in another empty seat. If it happened to me, I'd have to take a few seconds to get over the shock that someone had down that. It just not something I'd expect to happen on a plane.

Did the person pretend it was an accident? I'm just curious how he responded when you told him to move.

Yup, that is indeed how you do it. Unlike the person with whom the OP had been confronted, the man above really DID want to sit next to his wife. Imagine that.

Welcome to Flyertalk! What a great first post. You handled that situation so well especially with the way you spoke to the little girl. ^

I had just the opposite problem last year. I sat in the wrong seat -- right row -- wrong side of the plane. Middle seat, so it wasn't like I was trying to "upgrade". Guy comes up and tells me I'm in his seat. I looked at my boarding pass and said something to the effect of "OOPS, I'm sorry, I thought I was on this side" and immediately got up, grabbed my tote and purse and got out of the seat. He said "Don't you look at your boarding pass before you sit down?" rather snottily and I replied, "sorry, guess I'm dyslexic or something." He just wouldn't let it go, said "try to sit in your own seat next time". I apologized again and sat down on my side and I could hear him talking to the window seat passenger about how "some people" need to pay more attention to what they're doing. I considered offering to bang my head against the overhead or cut my wrists or something but just figured he was having a bad day.

Unbelievable! Some people are just generally unpleasant. Sounds like you dealt with it quite well!

How about this. I show up for my BA SEA-LHR an hour early (which is late) and wait while a couple of people at the counter struggle with the last couple of problem tickets. By the time I get to the counter they have giving away my seat. They seem to purposely take their time as if they want me to miss the plane. They tell me I should have showed up 2 hours early and call the gate to get a seat for me. I take my seat near the back of the plane in a middle seat of a 3-4-3 747 with no individual TV's. A woman approaches and has a boarding pass for the same seat. We compare our passes and the seat numbers match. The FA looks at the boarding passes, tells the woman to come with her and immediately wisks her up to the elite cabin...

We recently flew on UA from KOA to LAX. We were in first and my 14 year old son was in coach on an aisle. We deliberately put him in the aisle so he could get in and out without bothering anyone. When I looked back there was a man in the aisle, a woman with a lap baby in the middle and my son in the window. I went back and told them that I put my son in the aisle so he could easily get in and out. The man replied, "Well we need to be able to get in and out because of the baby." I replied, " That is my son's seat." My son said he didn't mind so I left it alone. Later, I noticed my son was in the aisle. I don't know if they moved because of me or if they realized it was not their seat.

I recently made a mistake in boarding and turned left instead of right. I had a great seat with a great view, neat headphones. Or so I thought - until the owner showed up. He was needlessly rude, I thought. I mean he might at least have let me try to fly the airplane.