DO4 tested versions: 4.3.5 build 6299 and 6420.

using (var transactionScope = Transaction.Open(transactionMode))
{
  using (var region = Validation.Disable())
  {
    int id = 1;
    Server server = Query.All<Server>().SingleOrDefault(server => server.Id == id);
    // THIS LINE THROWS EXCEPTION WHEN ENTITY 'Server' IS NOT REFERENCED
    if (server.IsReferenced())
    {
      ...
      // Do some work
    }
    transactionScope.Complete()
  }
}

This code trying to do some work when entity 'server' is not referenced. When entity is referenced (when IsReferenced() returns true) everythink is ok. But when entity is not referenced it throws such error :

Query task is not executed yet

with stack trace:

   at Xtensive.Storage.Internals.QueryTask.GetEnumerator() in c:\Temp\DataObjects.Net\Xtensive.Storage\Xtensive.Storage\Internals\QueryTask.cs:line 48
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Xtensive.Storage.RecordSetExtensions.<ToEntities>d__a.MoveNext() in c:\Temp\DataObjects.Net\Xtensive.Storage\Xtensive.Storage\RecordSetExtensions.cs:line 84
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Xtensive.Storage.ReferenceFinder.<GetReferencesTo>d__0.MoveNext() in c:\Temp\DataObjects.Net\Xtensive.Storage\Xtensive.Storage\ReferenceFinder.cs:line 71
   at Xtensive.Core.EnumerableExtensions.IsNullOrEmpty[TItem](IEnumerable`1 items) in c:\Temp\DataObjects.Net\Xtensive.Core\Xtensive.Core\Extensions\EnumerableExtensions.cs:line 46
   at Xtensive.Storage.ReferenceFinder.IsReferenced(Entity target) in c:\Temp\DataObjects.Net\Xtensive.Storage\Xtensive.Storage\ReferenceFinder.cs:line 55