Question:

Why does a user receive the following error: Insufficient Privileges?  This error occurs when the user tries to access the Sharing Detail page of a record.

Answer:

This is the expected outcome if the record's object sharing setting is either set to public read/write or controlled by parent.

When using public read/write, the record is available for every user.

When using controlled by parent, the record access is controlled by the parent record.

Related Documentation:

CRM Help Article: How to Determine Why a User Has Access to a Record in Salesforce.com (SFDC)?