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
