All those involved in the cryptocurrency community should know and understand the term zero-conf, or zero confirmations.
Zero-conf enables a customer to pay in cryptocurrency in a retail environment without waiting for at least one confirmation on the blockchain.
The reason this works is by leveraging Bitcoin's solution to the double-spend problem - the first transaction broadcast to the network is the transaction that the miners will pick up in the next block.
Consequently, Bitcoin has broken zero-conf by implementing Replace By Fee: https://en.bitcoin.it/wiki/Replace_by_fee However, using a Bitcoin wallet without Replace By Fee functionality allows you to still use zero-conf.
When sending your crypto to another user via a QR code, once the transaction appears (unconfirmed) on the vendor's wallet seconds later you simply walk out of the store.
I encourage anyone to experience this for themselves as this was an excellent experience for me at a restaurant where I did not have to swipe a card, sign or provide any of my identification information.
By leveraging this functionality mass adoption can be achieved in a retail/restaurant scenario.