CodeIgniter Action Requested Not Allowed

Environment: 
Site developed using CodeIgniter 2.x
Problem: 

When you submit a form on the site, you get an error message that states: An Error Was Encountered. The action you requested is not allowed. Other non-form actions on data (e.g., clicking a link to delete a record) work.

Status: 
Fixed
The Fix: 

This is a symptom of incorrect configuration of the CSRF protection built into CodeIgniter. Unlike others, I do not recommend simply disabling that protection. Why go without, or why reinvent the wheel? I use a framework so that I can have things done for me. The fix that worked for me was to ensure that a) the configuration is correct in your config/config.php file, and b) use the form_open() function in the form helper (which I have autoloaded, but that's not part of the fix, that's just me being lazy).

Fix License: 
Creative Commons: Attribution-ShareAlike