The average user has the same problems, but doesn't realize it is a UI problem or even think that he could be more productive. When he does he probably thinks that he is too stupid to use a computer.
The machine asked which payment option you wanted: cash, credit card, voucher, etc. She selected cash. That brought you to a screen with a button in the middle allowing you to switch quickly to pay with card (to pay the remaining balance after you'd part paid with cash). The button text was "pay with card". The problem is, because it wasn't obvious that it was a button, she thought she had accidentally chosen the wrong option at the previous screen. She chose the "back" button at bottom and then selected "cash" again, but had the same problem. I saw her go through this cycle several times before asking for help.
A really obvious button style could well have avoided the problem.
Edit: You could say something similar about any non-verbal cues. Why not say "error" instead of using a red error icon? Why not say "left" instead of using a left-pointing arrow? Etc. The answer is always that (if used appropriately) it lowers the user's cognitive load. Admittedly sometimes it's reasonable to use both e.g. usually you do say "error" in words too, and maybe your suggestion would also be useful for this button.