I've added a Taxonomy Field to my content type to collect tags from the Managed Metadata Service.
If I use 'Edit Properties' in the SharePoint Library I can add 50+ tags to my Document.
If I edit the document in office applications and edit the properties via the Document Information Panel I get an error and the document is unable to check in -
"To save to the server, correct the invalid or missing required fields."
Environment is SharePoint 2010 SP1 and Office 2010 SP2.
History is the Content Type used to contain a Lookup to another list but this hit the 50 lookup item limit, so we decided to use a Taxonomy field so we can add up to 250 tags.
I've checked through Technet and tried the couple of tricks users have suggested on other threads: changing the hidden fields to optional and back again, creating a new content type, creating a new doc library.