Security codes for Tokyo's main airport had to be changed just days before President Obama arrived, after an employee dropped a memo containing the top-secret details.
The Skymark Airlines employee mislaid the list of every security code on the departure lobby floor of Haneda Airport, an official revealed.
While the details were only missing for 30 minutes, it was deemed a big enough security threat that electronic security codes were changed.
The incident took place on Sunday evening, and was revealed by Japan's transport ministry.
Security in the capital was ramped up ahead of Mr Obama's visit, with 16,000 police officers deployed.
Japanese media said additional security cameras had been installed, and bins and public lockers swept for explosives then sealed.