Stunned passengers were left open-mouthed as they watched Madonna get off an economy flight - loaded with her Louis Vuitton luggage - after their plane was diverted on its way to Lisbon.
The music legend, worth around £600 million ($1.1B NZD), was flying from Heathrow with Air Portugal earlier this week when the crew told passengers that the plane had to refuel in Porto instead.
The American, 59, looked tired as she wore a long, black and white multi-checked coat, a monochrome striped scarf and hid her make-up free face under a grey fedora hat, reports Daily Mail.
She had been hiding completely unrecognised in the cheap seats - which cost from just £42 ($82 NZD) instead of £180 ($351 NZD) for business class tickets - until the flight landed in Porto and her agent whisked her through business class to the front of the plane.
An onlooker said: "We'd been on the ground for about 20 minutes when I saw her assistant speaking with crew members at the front of the plane.
"Then I noticed all this Louis Vuitton luggage being carried past me down the aisle too. And Madonna literally walked by me. I couldn't believe it.
"She had no make-up on but I Googled her and I discovered that she's recently bought a place in Lisbon, so her being on the flight suddenly made sense.
"I thought to myself 'Madonna is on my flight!"
"She looked tired as she sat and waited at the front of the plane for about half an hour. I could barely see her face, she was constantly turning away or hiding her face in her scarf.
"She didn't say or do anything while her agent was on her phone. Madonna was just sitting and waiting.
"Then she got off the plane and I saw her get on to an airport bus. I'd heard her agent saying that her car was waiting out front - they must have meant outside the airport."
The assistant appeared to have told the crew she was Madonna but they remained professional and didn't react. No one else appeared to recognise the Holiday superstar in their midst.
The lunchtime flight sat on the Porto runway for about an hour before flying on to Lisbon.
The source continued: "Then a lady asked why two women had left the flight at Porto. A female crew member whispered to her that it was Madonna. She was gobsmacked!"
The rest of the passengers eventually arrived in Lisbon three hours late yesterday evening.
When MailOnline spoke to Madonna's publicist Barbara Charone about this story, she refused to comment.
Air Portugal flights usually arrive in the city in 2 hours and 35 minutes, a quicker journey than it takes with rivals RyanAir and easyJet.
The Like A Virgin superstar now divides her time between Portugal, the US and the UK, where her eldest son Rocco Ritchie, 17, lives with his director father Guy.
Her son David Banda - who she adopted from Malawi in 2006 - joined Benfica youth football academy earlier this year, where his "prodigious talents" were said to have "impressed everyone" in his first week.
Madonna has now left the confines of a Lisbon hotel and has moved into her own home in Sintra, 17 miles from the Portuguese capital.
A local media report in August said: "Madonna is no longer a tourist, she now lives in Lisbon."
It came after news revealed that she was "uprooting" her family from their New York home to start a new life and to give her children "every opportunity".
It's believed that the actress has also enrolled David and her younger children, Mercy, 11, and five-year-old twins Stella and Estere, into the city's exclusive £30,000-a-year French Lycée.
A-list celebrities including Michael Fassbender, Monica Bellucci and John Malkovich have all purchased homes in the cosmopolitan city.
A spokesman for TAP said heavy rain in Lisbon meant there was bad air traffic over Lisbon.
The pilot was told that they would be in a holding queue for half an hour when they got there so they decided to touch down, he said.
The spokesman added that while TAP always carried sufficient fuel they decided that the better course of action was for the plane to land in Porto.
"TAP confirms Flight TP359 from November the 28, from Heathrow to Lisbon, diverted to Porto (distance between the 2 cities around 300 km) due to bad weather in Lisbon."