Glass.Mapper by default will check to see if a Sitecore item has a version in the language of the item you have asked for. For example I have an item that has 1 English version but no French version. When the code requests the French version class will return a null object because it sees that there isn't any French version. This is different to Sitecore's default behaviour which will return an item with zero versions, this can be useful for accessing shared fields.

To make class behave in a similar manner you need to use the version count disabler. This simplest way to do this is to wrap your calling code in a using statement:

using(new VersionCountDisabler()){

  var model =  sitecoreService.GetItem<MyModel>("/sitecore/content/home");

}

Glass.Mapper will now map the shared fields on the French item in the example above.

For some solutions it maybe desirable to always disable the check for every request. You can do this by setting the VersionCountDisabler up in the BeingRequest event of your Global.asax file and then disposing of it at the end of the request:

    public class Global : Sitecore.Web.Application
    {
        public Glass.Mapper.Sc.VersionCountDisabler VersionCountDisabler
        {
            get
            {
                return HttpContext.Current.Items["77D90B5A-236C-4116-8C4E-E4517753E74C"] 
                    as VersionCountDisabler;
            }
            set
            {
                HttpContext.Current.Items["77D90B5A-236C-4116-8C4E-E4517753E74C"] 
                    = value;
            }
        }

        protected void Application_BeginRequest(object sender, EventArgs e)
        {
            VersionCountDisabler = new VersionCountDisabler();
        }

        protected void Application_EndRequest(object sender, EventArgs e)
        {
            var disabler = VersionCountDisabler;
            VersionCountDisabler = null;

            if (disabler != null)
            {
                disabler.Dispose();
            }
        }

The Guid in the Items collection is a random Guid used to ensure that the the VersionCountDisabler does not collide with any other value added to the Items collection