This story goes WAY back to before cash registers turned into point-of-sale data-entry devices-sometime in the mid-1970's, I think.
I worked for a company that had its own cafeteria for employees. Everything was à la carte.
Entrées changed from day to day, but the cost was always the same. Likewise, desserts changed from day to day, but the cost of a dessert was always the same, and so on.
The lady who ran the cash register was very efficient. She had not only memorized all the prices, but she had memorized the totals of all the most common combinations of things. Coffee + donut, entrée + soft drink, entrée + soft drink + dessert, etc. Whatever the combination, she knew the total, and to save time, she'd ring up only the total, not the individual items. She was very accurate, and the line ran smoothly with practically no waiting.
Then one day the line suddenly slowed to a crawl. When I got to the cash register, after spending seemingly half my lunch break standing in line, I saw what the problem was. The company had raised all the prices. The cash-register lady, unable to rely on her memory, would stare at each person's tray for several seconds until she figured out IN HER HEAD what the total was. She had completely forgotten that the cash register could be used as an adding machine!