Why are global variables bad, in a single threaded, non-os, embedded application

Question

Most of the objections I see to using global variables make sense since they refer to issues of multiple threads, thread safety, etc.

But in a small, single threaded

Answer 1

The short answer is that global variables have historically been the source of subtle bugs. But then (putting on my functional programmer hat) all variables have been sources of subtle bugs.

Basically, a global variable means that you can't look at a piece of code using the variable's value, and know what it's going to do without knowing what every other piece of code setting that variable's value has done in the past.