--- app/models/issue.rb	2015-07-07 21:47:00.000000000 +0200
+++ /usr/share/redmine/app/models/issue.rb	2015-08-11 19:41:57.408794526 +0200
@@ -615,7 +632,16 @@
       if !valid_parent_project?(@parent_issue)
         errors.add :parent_issue_id, :invalid
       elsif (@parent_issue != parent) && (all_dependent_issues.include?(@parent_issue) || @parent_issue.all_dependent_issues.include?(self))
-        errors.add :parent_issue_id, :invalid
+        if all_dependent_issues.include?(@parent_issue)
+           # this would create a loop, don't like that! - new parent is a child or sub child of me
+           errors.add :parent_issue_id, "loop prevention #{@parent_issue} is a direct or indirect child of this topic"
+        end
+        parent_dependencies = @parent_issue.all_dependent_issues
+        if parent_dependencies.include?(self) 
+           # a child of this topic is also a child of the new parent
+           f=parent_dependencies[0];
+           errors.add :parent_issue_id, "an issue depending on the new parent also depends on this issue: #{f}"
+        end
       elsif !new_record?
         # moving an existing issue
         if @parent_issue.root_id != root_id