Originally Posted by Down_the_back
I once travled to Paris from BHX on my girlfriends passport! I took the wrong one by mistake! This was years ago, and after check-in went through the Euro Chanel, no one even looked at it. It was not until my return that I realized I had her passport.
Despite supposed tightened security, this exact scenario, reported by U.K. press, has happened very recently to a British national who took his/her spouse's passport by mistake for inter-E.U./non-Schengen travel.