It's not for a flight, but I once had this situation after a purchase :

I worked hard to save enough and buy a $1000 reflex camera, and I ended up saving $300 more so I added a compact camera to use for parties or bad conditions.

When I they processed my CC at the shop, it showed : "trouble with payment, take your CC back" on my screen, and the cashier told me "no, that's alright peyment confirmed.

After 3 months, (and now 4 years) I was never charged. $1300 worth cameras for free. I eventually bought a $700 lens for the reflex in the same shop, which was charged.

I think I was just lucky that the bug happened on me, and as for the shop, they didn't lose their customer anyway. I don't feel like I stole from them, as it just didn't work and that's life, and it is also a little amount disappeared on the million $ statements of the company, though I understand that it may bring some ethical doubts.

I would be happy to have my free flight one day, but I've had my chance already so I hope it'll happen to others, and possibly when they book premium!

I think this happened (or at least was more likely to happen) since you're in Japan.
Btw, I thought the Japanese were good at math. How could you have overpaid so many people?

All these stories of people not getting charged are pretty awesome. Sad for me since nothing like that ever happened to me

why came nobody to answer the most obvious question of all... how many miles were credited if any? you beat every cents per mile ratio

Several years ago, I had booked economy flights for my husband and I to go EDI-EWR-FLL. I was checking my booking online when one leg of the return journey disappeared so I phoned to query it and the agent said she would fix it and it would show online again in about 15 minutes. While I was on the phone, I queried how much it would cost to upgrade the EWR-EDI return leg and was told some outrageous amount. I immediately told the agent that it was far too expensive for me. However when I looked online next it was showing us in first class for the leg I asked about.
We kept waiting for the mistake to be rectified but it wasn't and we had a fantastic first class flight home - somthing we could never have afforded ourselves. The only problem is it has ruined travelling in economy!!

Just last week I noticed a couple of charges from a mom and pop restaurant hadn't posted. The total was probably only $50.

I did receive a "nearly" free flight. My inbound flight was delayed due to weather and I was automatically rebooked for the next day. However, I hustled over to the gate and made my flight home.

I was looking at my e-credits a few weeks later and noticed a fairly sizable one. I called in to ask what it was and we figured it out. I told the CSR I actually made the flight. She said, she couldn't remove it and it was mine to use. So, I used it.

Got a free webcam ($49 value) once. Ordered one from buy.com, and when the status showed as delivered, I still did not get the item. Had a chat with their customer rep. and was shipped another one. On the next day, the apartment management office called that they have a package by my name. The original item was delivered to another apartment and they had sent it to the apartment office. Received both the webcams on the same day and called buy.com - they said as the item has already been shipped, there's no need to send one back. Remained a loyal buy.com customer for long, even had a buy.com chase CC.

My best 'free' story was a few years ago eating dinner at Il Fornio in San Jose with a pretty lareg group. After a good meal and mutliple bottles of wine we started dessert and the fire alarm went off. No smoke so we sat and finsihed dessert until a couple of fireman came in and evacuated the place. We stood outside for about an hour but they never let anyone back in. I assume their insurance picked of the losses.

My local Home Depot installed those self-serve lines. I was one of the first to use them one morning when I bought some goodies for about $53. I put cash in and my change was supposed to be something like $17. the machine spit out about $92 in cash and a few coins. There were guys in ties watching the process so I told them that these machines are great, I'll definitely shop here more often. They were all smiles until I flashed my extra cash and told them what had happened. Their smiles dropped.
Apparently someone had put the bills into the wrong trays in the machine.

I always return extra change. Most of the time, it's going to come out of the cashier's pocket and I'd feel terrible if I took money from someone like that.
At Lowe's a couple of months ago, I bought a $90 battery charger along with about $500 in building materials. When the total came to about $100 less than my mental math, I pulled the cart over and checked the receipt. They didn't charge me for the charger. I recognized the opportunity to gut-check my 18 YO son to I told him, "check it out, they didn't charge me for the battery charger" he immediately said, "well, let's go back". I'm soooo proud of him. (He's in college to be a police officer right now.)

ROFL!

Perhaps the guys watching you were executives of Home Depot observing their customers try out the self serve lines.. only to find out, some employee fed the machine wrong..

They were probably doing a demo.. and turns out the self serve machine is like a slot machine handing out jackpots!!!

Actually, this happened at a major city bank in the US. It would NEVER happen in Japan since there are two levels of tellers and everything is counted at least 3 times.

FYI: Accounting errors are pretty common at American banks since most transactions are done one-on-one, but they do decrease with experience. Problem is summer tellers get so little they tend to screw up more.

Important questions:

1. Did you still accrue FF miles?
2. Where was your seat? Up front? Up front in coach? In the back in a middle seat surround by out-of-control kids whose parents were NOT seated nearby?