I encountered this error on a newly created site collection
Word Web App cannot open this document for viewing because of an unexpected error. To view this document, open it in Microsoft Word.
One look in SharePoint log files with ULS viewer did not reveal much:
w3wp.exe (0x1A74) 0x22F4 Office Web Apps Office Viewing Architecture b4vn Medium Conversion response received: CantFindSourceDocument 41e96340-f0a6-4b66-b471-83b9d2026d61
The problem was that service account which is used for Office Web Apps, in particular Word Viewing Service Application did not have proper permissions (db_owner) on the content database where the site collection is stored.
After setting the permissions, I got another cryptic error which fortunately I encountered before and was an easy fix
Word Web App encountered an unexpected error
To fix this, just close all browser sessions on the client computer and start over again.
