For me it just didn't print the ticket, so I couldn't prove I purchased it, fortunately I had my online banking app with me that showed the transaction.

I'd guess the worker putting the food into the bag is where the failure occurred.

I tried it on a lark just the other day, "mcdouble with no onions and extra pickles." Got the pickles but still had onions. /shrug

This will become better and normal very fast, and in ten years waiters and cashiers will seem as quaint as land-line telephones.

In China, it's apparently already very common to order (and pay) from your seat at a restaurant using an app.

In some restaurant chains in America, too. But we're talking about McDonald's specific kiosk system, not some random app in China.