When administering SharePoint Search in SharePoint Server 2010 (or 2013) you can create Managed Properties, or Metadata Properties, as it’s named in the Quicklaunch of the Search Administration. For a Managed Property you have to specify a name. When committing the input by pressing the OK button the name is checked for special characters. When you have entered a space in the name field, the input couldn’t be saved. That’s fine. But you are able to use other special characters, e.g. a hyphen. And with other character we had a lot of trouble. What does that mean? We can add these properties but we are not able to use them. When looking in the list of properties that come with the search service application, Microsoft only uses the underscore in the name of Managed Properties.
So, keep in mind: never use special characters in the names of Managed Properties, except an underscore.