If you're planning to head out of the country anytime soon, turn your cellphone off!

Joshua Fair of Atlanta learned this the hard way.

Fair went on a cruise that took him into international waters, locked his phone in a safe on the ship, but forgot to turn his phone on airplane mode, this little mistake cost him over $4,000 on his next AT&T bill. Turns out the device was still reaching for data thanks to apps running in the background like Facebook, e-mail, etc.

AT&T agreed to work with Fair and bring the bill down to $2,000, but still a huge chunk of change for a college student.

Moral of the story? Keep your phone at home!