In Django 1.9+ if you do not add: 'builtins': ['overextends.templatetags.overextends_tags'], to your TEMPLATES section you will receive a TemplateSyntaxError when the overextends template files are rendered: "Invalid block tag on line". Including 'builtins': ['overextends.templatetags.overextends_tags'], per the overextends docs (and experience) resolves this error.
https://github.com/stephenmcd/django-overextends