I recently had a problem where a user account on our test CRM installation was disabled without my knowledge.  The problem is I started getting authentication errors updating any records through CRM web services that had that user as the owner.  I could logon to CRM itself with the same credentials being used to call the web services and update record with that same disabled user as the owner, it would only cause problems udpating through the web services.  The error messages was just the generic authentication error which didn't help much.  So moral of the story is if you disable a user account change ownership to an active user on CRM entities.

PS - What I had to do to figure it out since I didn't know that user had been disabled is take a record that worked and one that didn't and go field by field in the database to figure out what was different.