This scam is used on passengers that make payments with cards instead of cash, how the scam works:
The card reader is connected to the driver's phone via Bluetooth, the driver changes the payment amount through his phone, the passenger is not aware of the change. A ride of a few euros can turn into thousands.
Payment with cash is the only way to avoid this scam.
Most frequent with taxis at Lisbon airport, Cais do Sodré train station, Pink street, but it's likely to spread everywhere else.
The news report is in Portuguese
Other scams involve:
Non working meter, don't use a taxi with a non working meter - it's definitely a scam.
Taxi drivers offering their services inside the airport, don't use them - legitimate drivers are at their taxis at the official taxi stand.
I use taxis frequently in Portugal, this what I do:
Only use taxis from the official taxi stand or phone the legitimate taxi dispatch.
Ask for an estimate before departure, if driver can't provide estimate I don't use them ( estimates have been accurate within 10 euros - I make allowances for heavy traffic, road closures, accidents,etc).
Meter must be working, it isn't I don't ride and choose a different taxi.
Only make cash payments to avoid the card fraud.
I'm also aware that some taxi charges are extra and are legitimate, day / night charge, day of week / weekend, luggage charges, toll booths, etc.
Bolt or Uber avoid some of these issues but they have surge pricing ( legitimate charge) and there some reports of some drivers asking for additional payment at the end of the trip - due to extra traffic, etc.
Older report regarding issues with Bolt, in Portuguese
https://portaldaqueixa.com/brands/bolt/complaints/bolt-burla-bolt-103968123