Wow--it would definitely be an awkward travel moment. The moment your captain gets on the PA system and asks for gas money to get to your travel destination. Huh? Thought that was covered in the cost of the airline ticket.
An Air France plane traveling from Paris to Beirut this week had to make an emergency stop in Syria because of security concerns in Beirut. While there, the passengers were asked if they had cash to help fuel the plane. Apparently it wasn't just a cash smackdown--there are sanctions on Syria and gas can only be purchased with cash.
Turns out the airline had enough cash to handle it themselves, and the plane traveled on to Cyprus and then on to Beirut. But in my case, I wouldn't have been able to help much. I carry little cash and use my debit card when I travel. The airline might have had slim pickings.