Is this a common Paris Metro Scam?

Nothing like that has ever happened to me on the Metro in Paris or anywhere else in Europe.

But if it did, I would insist we get off at the next stop and find a supervisor at the station.

There are official metro ticket inspections in Paris and generally, the fine is 35€ per person (more if paid later) for riding with an unvalidated ticket. If your friends are not exaggerating, what happened to them sounds like a scam assuming they were payng for one violation. If the people had badges and scanners, etc., they were most likely official ticket inspectors and perhaps somewhere along the way, the amount of the fine paid has been exaggerated. That can happen.

For those who are interested, here is the official list of RATP fines: https://www.ratp.fr/en/categorie-faq/4761?faqid=1616

Thanks for the link JHK, will download it onto my phone in case we are ever stopped

if you had 6 friends, maybe -- 6 people without tickets would pay 50 each. ONE ticket not valid is 50 though -- there is no fine of 300.

The OP says that at the Mom's metro ticket did not scan. That fine would be 35€ on the spot. It's possible that 6 people could have been traveling together and all were fare jumpers (50€ fine per person) but that seems somewhat unlikely and also is contrary to the initial post.

No exaggeration on the 300 euros paid. I have known this family for a very very long time and know their character. They would never lie about the amount. There is no reason for them too. The amount paid was not a problem for them. But it would be for me and my family. Just the 3 of them traveling.

After scanning your Metro ticket, be sure to hold onto it until you leave the station. Keep your used ticket separate from your unused tickets. Tickets can become unmagnitized, so keep them away from magnets and credit cards with magnetic strips. If this happens, you can get replacements from the info booth at a Metro station. If using a Navigo pass, be sure it registers when you tap it on the scanner as inspectors check those also.

Common, no.
But someone posted a year or two ago about "Metro inspectors" being anti-tourist, etc. Rather than anti-tourist inspectors, I smelled scam by impersonators. The scammers were in a less-frequently traveled hallway, outside the ticket zone where tickets don't need to be shown. The so-called officers wanted 60 euros, a bargain compared to your poor friends.

Is this correct: you are saying that the ticket went through the entry machine but the scammers claim it didn't demagnetize correctly--so cough up 300 euros. It's tough to say what one would do in that situation, but I'd hope I'd have the presence of mind to refuse and insist we go to the Metro office and if necessary insist they call the police. There is also a call button on the platform of every station.

However, this is easy for me to say because I speak French. (I bet these so-called Inspectors had accents from another country, but a non-speaker might not know the difference even if they were speaking English.)
If it happened to me in another country where I didn't speak the language, I could very easily be scammed the way they were. So sorry this happened.

Was it cash? What happens if you don't have 300 euros in cash? Sounds like corrupt cops

If your friends paid a fine of 300€ for one supposed infraction, they were well and truly scammed. Nothing to be done about it now but maybe consider it a lesson learned for your trip. These so-called inspectors must have seen your friends coming because asking for 300€ for that supposed infraction would have been a sure warning for many that this was a rip off in the making. It would be interesting to know if they zoomed in on your friends if other people's tickets were "inspected." If an inspector stopped me and told me that I had to pay a 300€ fine on the spot, I would have refused to pay and not just because I know the amounts of the fines but because I never have 300€ in cash on my person when on the Metro and would not have been able to pay. As Bets said though, it is easier to refuse if you speak the language. And, there is always the question of how conflict-averse one is or feels one should be when in another country.

With second hand information, it is difficult to fully understand and accurately analyze the situation. Métro inspectors typically work in groups, carry portable ticket readers, have visible identification, and wear dark green uniforms with a teal green trim. They can be anywhere in the métro network or on the bus.

I have not heard of a single account of ticket inspectors being impersonated by scammers. It´s theoretically possible but rather unlikely. I tend to believe that your friends, as honest as they may be, have a version of events which may be based upon suspicion and misunderstanding.

Métro inspectors typically work in groups, carry portable ticket readers, have visible identification, and wear dark green uniforms with a teal green trim.

This is correct. If you are ever stopped by the ticket inspectors, it is very obvious, and everyone around you is stopped too. They are kind of a swarm.

FWIW....last time I was in Paris we were transferring lines at the Pigalle station, and there was a random inspection in the middle of one of the corridors. I guess they were plain clothes transit police. There was no ID visible, armband or scanning device. I figured they were legit as both guys has holstered pistols attached to their belt.

I remember seeing a lone ticket inspector on the metro late one night, but he had a rather large dog with him. Doggie had a “don’t mess with me “ look about him.

A fine of 300€ because one ticket "did not scan" sounds more than a bit suspicious to me, especially after looking at THIS chart.

Don't carry one myself, but might be a good idea to start using the smartphone to take video of these people. If they're scammers, they'll immediately run away.

And if they are legitimate officials, they may get very angry if you start photographing them. You are not allowed to photograph public officials and workers in some countries. Once in Slovenia, I was taking pictures off the train platform when it was stopped. A Slovenian train worker got very angry because he thought I was taking his picture and demanded to see my pictures and make me erase the ones of them. There were no pictures of him or the other worker, so I didn't have to erase any. (Nor did I get an apology.)

Doesn’t the scanner print something on the ticket when you go through?

The mom scanned her ticket, the gate opened, she went through, so she thought it was validated. They were stopped at the next stop and told she had not validated her ticket and went through illegally. Was told they had to pay 300 euros right there.

There is usually a faint marking on the back of the ticket but recently (actually in May 2018), there was no stamp on a ticket I used to enter the system at Bastille station. We had an inspection at Franklin D Roosevelt station and I had no problem so I think the scanning device that the inspectors use does not rely on the stamp.
As to the mother whose family paid a 300€ fine for a single unvalidated ticket, if it happened exactly as described by the OP, I call scam/ripoff. I have not heard of phony ticket inspector scams being run in the Paris metro (and I recall the post that Bets is referring to) but things change all the time.

Never happened to me, not even close in all my trips to Paris since 1973, and I ride the Metro at least once or twice now on every trip although I am beginning to rely on buses instead of the Metro based on the last two trips. In the past I relied on the Metro exclusively...still, nothing happened.

Have I used a Metro ticket on the bus or Metro that didn't work? Very rarely, but usually a canceled one mixed in with the unused ones. Then I pull another one out to insert, which works.

You don't rely on just one ticket ticket, at the bare minimum I carry 5-6 of them, the good ones in one pocket, the canceled ones go into another pocket in case I get checked, which also does not happen.

There are also times (though very unlikely) when I am not carrying 300 Euro on my person...what then?

Yes, from what you've described in your OP and subsequent message, and from what we know about the real fines for traveling without a ticket in the Paris metro and the way that checks are conducted, it seems clear that unfortunately your friends got scammed by some unscrupulous individuals apparently having nothing really to do with the RATP. This isn’t, however, something I’ve heard of happening before so I would venture to say it is rare. (But not rare enough if these thieves are victimizing people like this!!)

What would be the best way to respond in the situation the OP presents, as a tourist who speaks perhaps only a little French and is unsure if these are real Metro Police or not? Would it help to ask (as best one can) for clarification of the fine, perhaps utilizing a smartphone screenshot of the list of current fines, i.e., letting them know that you know what the fines should be? Or to say you don't have 300 euros on you? How are the fines paid in the case of real officers stopping everyone and issuing fines--is there any paperwork involved, and what happens if you don't have the money for the fine on you?

They may not be impostors, but could be REAL inspectors working for their own pocket. What do you think?

Suspicion upon suspicion My poor brain reels. I guess the only way to avoid all these phoney ticket inspectors, real ticket inspectors but possibly dishonest ones, bus drivers with hard hearts, etc., is to a) walk everywhere b) use taxis or c). Rent a car.

We have often seen those sleezey guys jumping the stile to enter the metro, and the same guys trying to hide from the inspectors on the train. Thus the need for the RATP to employ inspectors and sometimes they confront a poor innocent tourist who cannot show proof of payment. And maybe from time to time the tourist is not innocent at all.

Walking everywhere is indeed the way to go, but unfortunately many travellers are too old to walk much.

https://www.theparisreview.org/blog/2013/10/17/in-praise-of-the-flaneur/

https://en.wikipedia.org/wiki/Flâneur

The only city where I could count on being "controlled" by the ticket inspectors every time was in Budapest, ie, in each of the three visits there just before entering the subway platform. They (2-4 ) would be at the bottom of the long escalator waiting for you...no two ways about it, no at random checking. You'll be checked...period.... never had a problem with these BP controllers.

If the Paris Metro ticket you are using does not work, time to buy another one, spend that amount to exit just as I did trying to exit coming back from St Germain-en-Laye. It didn't work, so I got another one at the ticket counter.

The one time I was fined, not knowing if it was a scam or not, was in 1989 riding a German train where the controller didn't accept the rail pass. All other German conductors prior to him and after him on the same trip (4 weeks that summer) did accept the Pass on the same type of train too.

He said I had to pay in cash, forgot the exact amount now, and I didn't know why. This caught me by surprise, argued with him back and forth in German, didn't bother to ask if he spoke English or not. Chances are he would not have, don't recall if he saw my passport which I had on me, and thought I was another dumb American not knowing about German trains. This was my 6th trip in Germany since 1971.

One thing I knew prior to traveling in Germany in 1971 is that one doesn't argue with "officialdom" (Beamtentum) and get away with it. They're not going to yield. After the first minute or so, I knew I was going to have to pay in the end, that he would not give in.

He's standing in the my compartment going back and forth with me, I decided to waste as much of his time arguing and countering his arguments, ie strategy of attrition.

What I should have done was to demand to talk to his supervisor/boss, (Vorgesetzte), ie, someone with more authority than he. I don't know how many minutes I ended up wasting his time before I told him I would pay, he wrote out a receipt. Of course, I didn't think of inquiring at the next Reisezentrum.

My experience is the ticket checkers hang out in the hallways of stations (generally behind bends where you don't see them until you are right on them) and scan everyone who comes by - not just tourists. If you're in the metro, you need a valid ticket even if you aren't on a train.

No idea what fine they charge but it's a good reason to keep track of your ticket. My technique is to keep unused carnet tickets in my left pocket, my current ticket in my right pocket and get rid of used tickets right away so I don't confuse them.