It hasn't happened for awhile, but there was a time when supermarket checkout barcode scanners were were slow and unreliable. Actually, it still happens in Dan Murphy's, where the number related to the barcode sometimes has to be manually entered into the register. The numbers are many digits long, yet the check out chick or chap glances at the numbers and seems to be able to enter them all into the register with one hit. Amazing. I can't recall barcode digits being manually entered in supermarkets for quite some time.
I am sure it is a thing you can learn. I have tried with my bank account when I make a transaction and write down the record, but I can't get much better than six or seven numbers, however it is an improvement on my past three numbers at a time.
I really wish I knew someone who worked at a supermarket check out to ask how they remember so many bar code numbers. Perhaps the first few digits are always the same.