Comment by rmunn

The counterexample I've personally seen is multiple people editing the same file with different editors. The first person has his editor configured to indent with tab characters, and he writes Python code like this:

    def example():
    <tab>if True:
    <tab><tab>do_something()

Now a second guy edits the file. His editor is configured to indent with spaces. He adds `do_something_else()`, and doesn't notice that the code block no longer has a consistent prefix:

    def example():
    <tab>if True:
    <tab><tab>do_something()
            do_something_else()

Notice that because I've used five characters to type `<tab>`, it is already visually obvious that this is incorrectly indented. But that wasn't obvious to guy number two, because this is what he saw on his screen:

    def example():
        if True:
            do_something()
            do_something_else()

The compiler itself isn't per se concerning itself with how different editors have chosen to display tab characters. But in practice, it has to decide "is the do_something_else() line part of the `if True` block, or not?" And so it has to have some opinion on tab characters. Here, that opinion will be "Inconsistent mixing of tabs and spaces in same file, impossible to know programmer intent, refusing to guess; raise TabError exception here".

The use of .editorconfig files should, in theory, solve this. But just yesterday I had another file, thankfully one where whitespace was not significant. The .editorconfig file said "Indent with tab characters", so my editor, when I opened a new line, indented it with tab characters. But the file was actually indented with spaces, and nobody had fixed the .editorconfig file to say "indent with tab characters... except for this file which is indented with spaces".

Editor misconfiguration in both cases. Not strictly the couterexamples you were asking about. But the Python example, although made up, is reflective of actual situations I've seen. People with different editors editing a file, not paying attention to whitespace, and ending up with ambiguity where the width of a tab character would actually make a difference to whether a line visually appears lined up with its indentation block or a different one.

Tabs are great for indentation in theory. In practice, I've personally seen more pain than gain from files that used them.