Your boarding pass for a flight can sometimes be used to get your passport number. Don’t post your boarding pass or baggage receipt online, keep it as secret as your passport.
How it works
The Booking Reference on the boarding pass can be used to log in to the airline’s “Manage Booking” page, which sometimes contains the passport number, depending on the airline. I saw that Tony Abbott had posted a photo of his boarding pass on Instagram, and used it to get his passport details, phone number, and internal messages between Qantas flight staff about his flight booking.