Luckily, the 36-year-old actress was saved from any embarrassment by her bra - although its flesh colour had several fellow passengers doing double-takes yesterday.
That white racer-back vest might be comfortable for travelling but at times like this has a less than classy look.
Oops: Gwyneth Paltrow's baggy vest exposes her bra as she goes through security at Barcelona airport on Sunday
A couple of minders were on hand to make sure no one got too close.
Gwyneth was at the airport with her children Apple, five, and three-year-old Moses.
They were heading home after watching her husband Chris Martin play in Barcelona with Coldplay on Friday.
A fluent Spanish speaker, Gwyneth doesn't always follow Coldplay on their world tours, but jumped at the chance to see her husband in one of her favourite cities.
Having spent time in Spain as a teenager, Gwyneth is making sure her kids grow up bilingual.
She explained: 'Moses speaks a little bit, but understands everything. Apple speaks a lot.'
Such is her love of the language, Gwyneth has launched a Spanish version of her lifestyle website Goop.com.
She is also hoping to show off her skills in a Spanish movie in the future.
Peek a boob: Gwyneth's scarf still doesn't cover her bra as she chats to a security employee
She said: 'I would love to. I haven't found the script yet, but the first (movie) would have to be a small role, not very important, because I imagine I would have to get used to acting in Spanish.'
While Gwyneth returned to London, where the couple are based, her husband was spotted jogging with a minder through the centre of Barcelona.
Wearing black jeans, a T-shirt and trainers, the rocker worked up a sweat under the watchful eye of his burly guard.
Gwyneth is currently enjoying being a full-time mother to the couple's children after she finished filming Iron Man 2 earlier this year, which is due to release in May 2010.
Keeping trim: A night after his performance, Chris Martin felt the burn as he jogged in Barcelona with his minder
