I was in Paris in April 2016, trying to purchase metro ticket at a vending machine to Versaille Palace, a person offered to help, he pressed several button on the screens and showed 97 Euros for 4 round trip tickets! I remembered Rick Steves' book says about 7 Euros per ticket, I immediately was alarmed this guy was probably trying to pull a scam so I proceeded to the window and sure enough 7 Euro per round trip ticket. I was sure what kind of scam the guy was trying to pull, but thank Rick's book to avoid the scam.
That scam seems to be popular, and one reason I don't let anyone "help" when I'm buying tickets. Thanks for the reminder.
The way that this one goes is that he offers to pay by card and take your cash (if you try to pay directly with a card you are told that it is no good as the machine will only accept French cards). He then doesn't actually buy the expensive tickets (although will again press a load of buttons followed by cancel) but gives you 4 tickets he has already bought pocketing the difference. These tickets might open the gate but may not be valid all the way to the destination.