I'm developing in Visual Studio 2010 and it seems that the port used for the development server is causing the problem because when I change it to another random port, the problem goes away. Not sure why though, just that changing the VS development server port fixes this issue.
Changed this in the .csproj:
<DevelopmentServerPort>49288</DevelopmentServerPort>
To this:
<DevelopmentServerPort>53845</DevelopmentServerPort>
fixed it. Or you can just change it in the project properties. No idea why though, other than that VS2010 must of somehow corrupted that server somehow.