This is here mostly to give me a concrete example of this happening.
I have a directory, with two files in it, ab.txt and a-c.txt. Which sorts first when I do a ls? It depends on what locale is set. Like this:
Same thing happens with the sort command:
I found this surprising when it first broke some code I was using, since I'd spent so much of my existence in LANG=C (I'm still surprised that Unicode has existed for more than half my life), but once I set LANG=en_US.UTF-8 because I was using UTF-8 characters in my documents I found I was in a different (sorting) world.