After installing Office Web Apps bits and configuring Word Viewer and PowerPoint service application in your farm, you will get this error when trying to open a Word document or PowerPoint PPT file via browser:
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Looking at ULS you should see an entry similar to this error:

“SQL Database 'WSS_Content' on SQL Server instance 'SQL Server Name' not found. Additional error information from SQL Server is included below.  Cannot open database "WSS_Content" requested by the login. The login failed.  Login failed for user 'Contoso\service'.”

In this case, Contoso\service was the service account used for Office Web Apps service applications, after adding the service account as dbo to the content database hosting the document I was able to view and edit the document via browser.