WARSAW CONVENTION
Chapter II - Documents of Carriage

2. The absence, irregularity or loss of the passenger ticket does not affect the existence or the validity of the contract of carriage, which shall none the less be subject to the rules of this Convention. Nevertheless, if the carrier accepts a passenger without a passenger ticket having been delivered he shall not be entitled to avail himself of those provisions of this Convention which exclude or limit his liability.

When a passenger books a flight, a contract occurs between the passenger and the carriage. Even if the passenger ticket is lost, the contract still stands. However, if for any reason the airline accepts a passenger without a ticket, the airline cannot use the Warsaw Convention rules that limit or exclude its liability.