I have been getting this error quite a bit recently:
<authentication mode="Windows"/>  
The full description is here:
Configuration Error Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level.  This error can be caused by a virtual directory not being configured as an application in IIS.

Source Error:
  Line 24:             ASP.NET to identify an incoming user. Line 25:         --&g ...