Overview
When clearing the cache in Drupal, the following error sometimes occurred.
Here are my notes on how to resolve this error.
References
The following was helpful.
Solution
A module named user had been created by the Features module.
/modules/custom/user
Deleting this resolved the error.
Additional Note
Similarly, a module named comment was also causing issues. Deleting it likewise resolved the error.
Summary
I hope this helps others experiencing the same error.