General error when creating an account
The field_data_field_name_first field had some data stored in it for that user ID, apparently because it was migrated in at a later time than the user table or because I didn't drop the database before I imported new data. As a result, the system couldn't write to the database user table and field_data_field_name_first table. The solution was to do a fresh import of the user table and the first and last name custom fields.

This was on a test site.