In the documentation, it is said:

    Indentation is rejected as inconsistent if a source file mixes tabs and spaces in a way that makes the meaning dependent on the worth of a tab in spaces; a TabError is raised in that case.

But that's not true. For example, Python thinks that these two indentations are "consistent":

<tab><8 spaces>
<8 spaces><tab>

However, their width would be different for any tab width except 1, 2, 4, and 8.

Actually, it's not easy to check that indentation is "consistent" as it is defined currently, so I think that it is the documentation that should be changed. So, I think that the paragraph that I quoted above should be changed to match the actual behavior.