404 Error in IIS for a Url with a plus + in the Path

Default Internet Information Server (IIS) behavior since IIS 7 rejects requests to Urls with a plus (+) sing in the path like:

www.somesite.com/one+two or
www.somesite.com/path/subpath/three+four

The behavior is considered a security feature and controlled by a setting called "Double Escaping Filtering". To override the default rejecting behavior the setting needs to be changed either via a web.config configuration:

<system.webServer>
  <security>
    <requestfiltering allowDoubleEscaping="true" />
  </security>
</system.webServer>

or an IIS Management user interface: