Joomla on times can show you the following error upon creating a new article or editing an existing on in article manager.
Save Failed with the Following Error: Invalid Parent ID
This is appliable to Joomla 1.6, Joomla 1.7, Joomla 2.5 and Joomla 3+. Since Joomla 1.6, #__assets table was introducted which saves a foreign key for all the Joomla's core (and in some cases 3rd party extensions) content.
To solve your problem. follow the steps below.1
1. Go to the Menu Manager, select all menus items and click the rebuild button.
2. Go to the Category manager, Select all categories and click the rebuild button.
3. Open all categories and save them (this will insert the approriate assets tables associations for all categories).
Next time you go edit your article, You won't see the error message again.