Programmers, can you spot the error in this Wordle program?

As a programming exercise, I coded a function that would give the letter colors for a guess of a particular word. The program is not that long but it is not trivial.

I went online to see if I could find a similar program. I found this. You can see the code if you scroll down a little. Can you spot the error and give a counter-example that will not work?