As an example mako cli. You can call this an error or not, but with C locale your cmdline will die with UnicodeErrors when you open a non existing file with unicode filename on Python 3 but not so on Python 2 where it will do the correct thing. It will also die with unicode errors under the same situation when your template renders any unicode characters. Again, something that probably works fine on python 2 and correctly.

Or if you would put unicode characters into your README.rst you could no longer safely install mako. Again, Python 3 only.

These are just two things I found on github.

Another easy one: alembic README's now no longer can safely contain unicode. They would break on Python 3, but work just fine on Python 2 because of the code in list_templates.

the cmdline template runner at the moment isn't doing unicode in Py2K either, crashes there too.