After upgrading SharePoint Server to SP1 or later, you are no longer able to view Word or PowerPoint documents with Office Web Apps Server 2013.
Office Web Apps give the error of "Sorry, we ran into a problem." and gives the options to Restart, or Exit.
Software combination scenarios we see the issue:
Internet Explorer, Office Web Apps Pre SP1 and SharePoint SP1 or greater
Update Office Web Apps Server to at least SP1 (http://support.microsoft.com/kb/2880558)
*Note you must follow the process below to apply updates to Office Web Apps 2013