A quick (or not-so-quick) aside: when traveling internationally, I have learned always to carry a printed itinerary and receipt with me.

Last year, I flew JFK-IB-MAD-IB-AGP-IB-BCN-IB-MAD-IB-SCL-LA-MIA-AA-BOS. The SCL-MIA flight had a stop in CCS, and the plane was taken out of service and the CCS-MIA segment cancelled because of a mechanical issue. After sleeping in the CCS airport for the night, in the morning the AA desk was a mob scene, as all of the displaced passengers tried to get on AA's morning CCS-MIA flights. LA was of very little help. Ultimately AA agreed to accommodate everyone, but required passengers to show a receipt. When I got to the front, I explained that I had booked through AA and that they would find everything they needed in the PNR. To my amazement, they insisted that they needed a printed receipt. Fortunately (and by pure luck, since I don't normally carry one), I had a print out of my receipt. I then had to spend the next 15 minutes convincing the AA agent (yes, this was an AA agent, not a third-party contractor) that a receipt printed off of the internet was the same as a receipt printed on airline ticket stock.

It was an unbelievably frustrating experience. And so while I know that the odds of actually needing a receipt again are very, very low, I now make it a habbit of carrying one with me on international trips. (And if I think of it, I'm gonna start asking the AC to reprint the itin and receipt for me on ticket stock!)