Interaction designers - Pay attention!
This is a picture of the control panel of a vending machine at my work. There are letters and numbers. Every piece of candy in the machine has a unique letter number combination (A1, G9, etc.). There are 40 pieces of candy in the machine, thus you need 40 unique pairs of letters and numbers to be able to select all the candy.
In this interaction, users scan the machine then zero in on the candy they want. They read the candy ID then head over to the machine to type in that ID with the fear that a mistake will get them stuck with some dreadful candy. Seeing that there are letters and numbers, the mind thinks that this is a 2-step interaction - type the letter... then the number.
But wait - in this vending machine they've assigned two digit numbers to some of the candies! So that's a three character/digit ID in some cases!
So what do you think happens when a user sees the beloved cashews with ID D10? They type in D-1-0. But because there is a "10" on this panel, user is stuck with nougat-filled Snickers. Not really satisfying, and not what user wanted.
The designers meant well - they wanted each row to start with a unique identifier. And they wanted candies to be numbered sequentially in each row. But that's not how users interact with the machine. We're not counting rows from the top, or counting how many candies from the left our beloved 100 Grand is. We're looking at the candy ID code. And including double digit numbers is seriously confusing.
Suggestion: Use numbers for the rows and letters for the columns (1A, 8Q, etc.).
I feel better now. But I hate Snickers.