A very similar thing happened to me on a Qatar Airways flight once. I was seated in J and the forward lavs were occupied, so I went backward to the rear J cabin. A FA working that cabin intercepted me and told me these lavs were for J and I needed to go back to Y. When I explained I was ticketed in the forward J cabin, she was mortified and apologized profusely.

I have a friend who made a habit of sneaking into domestic F. He said it worked a surprising number of times. I would never have the guts to do that.

I don't believe it works.

It doesn't have to work all the time, or even most of the time, to be worth trying.

Possible downside: An FA tells you to go back to your seat. You go, and that's that. No further repercussions. Nobody charges you extra, puts a note in your permanent record, or anything at all. No damage done.

Possible upside: You get a free seat in F for a while, perhaps for the duration of the flight.

That equation doesn't create much motivation not to try it. Now that I think about it, I'm surprised more people don't.

Other possible down side is getting billed by the airline for the seat you poached.

Only did it once....
 

....back in my backpacking days.
Was on an Air NZ flight to Auckland...flight wasn't that full...so I took the very back seat in J (not lie flat). Managed almost 3 hours of sleep before I was noticed. FA was quite nice, she just said sir is this your seat, I just said no I took it because my seat mate was snoring and I was exhausted. She just asked me to move back. That was it. Never tried it again.

Back in the early 90's I was on a flight from KJP to YYZ on AC. I was in F. About an hour into the 5 hour flight a man comes up from Y and plops himself down in an empty F seat. It was maybe half an hour before an FA came up and asked to see his BP. Obviously, he didn't have one for F.

He absolutely flew off the handle when confronted, and played the DYKWIA card about how he flew so much on AC that he should have been UG'd, and that he was sitting there weather they liked it or not, he was going to report them to their supervisors etc etc etc. Further pressure from the cabin crew made him start swearing, using profanity and uttering what were threats to slap them etc. It was F you this and F you that.

Keep in mind, this was in the early 90's, not today..when such actions would get a flight diverted and S.W.A.T boarded.

Instead, they actually let the guy sit there and continued the flight. He didn't get any service of course. I was miffed that it looked like he was going to get away with it. However, upon landing at YYZ he was met by Toronto's Finest in the jetway and escorted out of sight. The odd part was that he was totally quiet and compliant with the police and seemed resigned to being arrested.

I can only assume the charge was "theft of services", although his threats to hit the crew were probably a large part of the reason. Air Canada does have a known history of having seat poachers charged. They did it to a woman a few years ago as well.

Extremely unlikely without a prior request to move back and a statement along the lines of "if you stay there, we'll bill you for the fare difference."

This is more like what happens. If you comply, as HomerJ presumably did, that ends the matter.

My story
 

Now I think my experience was probably okay but yeah still could be seen as not being completely honest.

I was flying in 2012 Amsterdam to Taipei on China airlines (CI).

In those days they were still flying their B747's.

This flight actually flew from Amsterdam to Bangkok and then on to Taipei under the same flight number.

So when I arrived at the gates. Which I was late as I enjoyed myself too much in the lounge plus I didn't realise they weren't making boarding calls.
I given a new boarding pass AMS-TPE in business class seat 04K or something like that in the very front section (No first class on this flight).

I can say I enjoyed my 12hrs or so and had the seat next to me empty which was great.

Now the guy mentioned at the gate (In Amstersdam) and wrote on the back of the boarding pass in pretty small writing that I would be back in economy for the BKK-TPE section.

I didn't care it was my first op-up without using my points.

Anyway when we got to BKK and went to re-board the plane again they scanned my boarding pass. Obviously the scanner gave a red beep as it didn't understand this new boarding pass. The staff looked at it for a few seconds and had a quick chat with another other staff, then just directed me to a another business class seat in the mid-section this time.

I knew what had happened but I didn't say anything and they didn't want to investigate too deeply as I suppose their thinking was... if I was actually a paying business class passenger I might be annoyed them stuffing me around..

So yeah I had another 4hrs of stretching out. Also had a Japanese diplomat sitting next to me I believe the deputy ambassador to Thailand. We shared a Sapporo beer or two and had a great chat. He was on his way to visit the Taipei Japanese trade mission (not allowed to call it an embassy).

Anyway I have had a good 10/12 op-up's (All international) since then but nothing like that experience where I sort of was a little dishonest in not saying anything when I could have.

I felt on a whole I could look at myself in the mirror and be okay with what happened.

I was on a TK flight from IST to FRA in January. I was in 3B on a 321 in FC. An older guy shuffles past the first few rows pretending to be looking for 'his' seat. Finally settles down in 3F with no one next to him. Two guys arrive and look at the seat, check their tickets and then ask the gentleman to check his ticket. He glances at his ticket and then grabs his junk and asks to sit next to me. As he takes his seat I realize he is not only as drunk as a skunk but also had some mental issues. I told the lead FA that I believe the guy was not fit to fly, he really did stink of booze and had 2 more alcoholic PDB's befroe pushback. The FA pretended not to understand and in the end I moved forward to 1F as I was unhappy with the constant physical contact, such as touching me on the arm, muttering etc. I did catch a glimpse of his BP, which was 24A (not 4A). So in this case he got away with it, due I believe to the FA's not wanting to make a scene.

I was in J on an international flight (probably LAX-LHR) and a lady "passed out" in the back about an hour into the flight. They asked for doctors or nurses on the flight and soon after the lady was brought up to J and stayed there for the remainder of the flight. She slept a few hours then was soon watching movies, had meal service, the whole thing. Was up in J next to me for about 10 hours I would guess.

I'm not one to judge, but she seemed to be OK.

Note to self: If you ever go for a midflight walk to the back of the plane, either a) let the FA know or b) keep your boarding pass with you.

I wish I had that QR crew. Last time I was on a QR long haul flight in F they let these screaming, annoying Qatari kids who were with their parents in J come forwards to use the F lav, while making noise and a scene. They would periodically run into the F cabin also, FA's never did a thing. Disgraceful.

No QR FA would ever stop that. If it's Qataris involved, they would be more worried about losing their jobs for saying anything than whether you might be annoyed.

The only way it would have been stopped would be if you had a qatari with even more wasta in the F cabin get involved. Welcome to flying with the ME3, where some are more equal than others, and there's nothing subtle about it.

Flying MUC-MAN with LH a few weeks ago. Got upgraded at the gate to 2D and 2F. Boarded and there was a woman in 2F. Told her she was in our seat and she just said "No". Asked to see her BP and she just ignored us. Got the FA and she demanded that the woman move (in German). Couldn't understand most of it but it did sound like she felt she was entitled because she was old (Ältere?). I think she was threatened with arrest because she eventually moved but was wagging her finger at the assembled FA's and my travelling companion and me and pontificating (still in German).

I was offered a free trade to Club from WT on BA PHX to LHR last year by a love sick girl who wanted to sit by her true love, so maybe $2,000 of free upgrade value. The FAs glared at me during service, zero smiles, but they never said a word even when I buzzed them for more champagne - maybe that was just how they were anyway.

As for sneaking into F, I have not seen an internal first section with empty seats for a loooong time - too many frequent flyers with status for that to happen, and TATL flights greet you by name when they take your coat, so good luck with that.

Honestly I would be too embarassed if caught, and too uptight to enjoy it even if I wasn't. If you want it that badly then cough up the shekels, otherwise put up with puking babies and exciteable kids in the back of the bus.