Trying to track down an error that's popping up in our php logs --
PHP Fatal error: Call to undefined function get_template_part() in ...index.php
It shows up 2-4 times a day (while index.php gets loaded dozens of times a minute). Even if we look locally, with no caching, we can't find what particular page or usage is firing this error.
I'm guessing that there's some particular pageload or something that's causing this... but I might be wrong.
Gurus! How do we track down what might be causing this? I'm looking for a repeatable Wordpress/php troubleshooting workflow, if possible.