The downside of electronic tickets is when the airline can't see them and they are lost in cyberspace! This has happened 5 times to me now, and I've learned to always have my ticket number with me as well as the PNR. Even so I've been unable to take the flight that I wanted several times because it took too long to straighten out the computer mess that made the ET disappear! Very frustrating -- and in those situations a paper ticket would have been painless and I would have made the flight instead of missing it!!! I am a big fan of ET but the industry needs major work to make the ET more reliable, and not put the onus on the passenger to prove that they have a ticket and are not commiting fraud.