Rebooting didn’t work for me, so I’d like to share what did work.
First, I just upgraded to Windows 8. This problem didn’t occur before then. And this issue only happened in one of my app.config files. So I decided to compare the problematic app.config with a good one. In Visual Studio, with the app.config open, I went to Xml -> Schemas. I noticed that the good config only had three of these schemas checked. The bad one had the same three checked, plus a DotNetConfig.xsd. After I changed the Use column to automatic, for DotNetConfig.xsd, the problem went away.
