Chris Petrilli notes that a network outage can baffle the staff:

Today, I had lunch with a friend at The Daily Grill in Bethesda, MD, and when I went to cash out our check, the waitress came back with a “your card has been declined.” Now, I know how much American Express loves me, and my constant stream of payments, so I figured that wasn’t it. When I looked at the receipt, it said “Rejected: NO CARRIER.”

I've seen this with cab drivers too - after they drop me off, they get downright odd when they have trouble getting a connection to the authorization server - and I know that they have the old carbon-copy devices in the car.