When having added a custom taxonomy through your functions.php in your theme, you find yourself not being able to add any tags to the custom taxonomy in either the post or the tags screen in the admin backend. And you are getting a "An unidentified error has occurred." error message in the tags screen of the admin backend when trying to add a tag.
The following could be the reason:
The taxonomy field in the term_taxonomy table allows for 32 characters. If the name of the custom taxonomy is longer than 32 characters the name of the taxonomy will be truncated.