25 or so years ago we were designing one accounting application that required intensive input of numbers by operators.
That was in Russia at the time when personal computers were just appearing in offices massively.
Amount of numeric data that operators needed to input was enormous and as in any financial institution the requirement for correct input was very high.
Operators were quite professional and were doing very fast blind input by hands at the time when their eyes were focused on paper sheets with data.
In order to help them to verify their input I invented and implemented “musical” verification. When focus lefts numeric field, the application is playing melody where each digit has its own tone. I was surprises how effective it happened to be. At the end of first day operators were able to determine at least number of digits filled and after some time some of girls were even able to tell exact number by only hearing the melody.