Last week I've been installing Microsoft Office Web Apps for my client. The client is running rather smaller farm with 1 WFE server and 2 application servers.
After running installation packages on all servers I went to one site and what was my surprise... I got Internal Server Error 500.
In few minutes I found out that installation corrupted ALL web.config files of ALL SharePoint web applications, specifically it duplicated staticContent tag so it looked like this:
<mimeMap fileExtension=".atomsvc" mimeType="application/atomsvc+xml" />
<remove fileExtension=".svg" />
<mimeMap fileExtension=".svg" mimeType="image/svg+xml" />
When I removed the duplicated tag, the site started to work again. However I had to update 15 web.config files (5 web apps, 3 SharePoint servers). This could be a serious problem if you are running middle-size or large-size farm with tens of web apps.
This is just another reason why web.config files should be a part of your SharePoint back ups.