diff --git a/include/wx/sizer.h b/include/wx/sizer.h
index 967a49ff61..324aee4301 100644
--- a/include/wx/sizer.h
+++ b/include/wx/sizer.h
@@ -1077,6 +1077,11 @@ protected:
     virtual wxSizerItem* DoInsert(size_t index, wxSizerItem *item) override;
 
 private:
+    // Set to true if any items _not_ using the associated static box as parent
+    // are added to this sizer: this is still supported for backwards
+    // compatibility but is not recommended.
+    bool m_hasNonBoxChildren = false;
+
     wxDECLARE_CLASS(wxStaticBoxSizer);
     wxDECLARE_NO_COPY_CLASS(wxStaticBoxSizer);
 };
diff --git a/interface/wx/sizer.h b/interface/wx/sizer.h
index bc66c27ef4..904019dcda 100644
--- a/interface/wx/sizer.h
+++ b/interface/wx/sizer.h
@@ -1944,8 +1944,12 @@ public:
 
     Note that since wxWidgets 2.9.1 you are strongly encouraged to create the windows
     which are added to wxStaticBoxSizer as children of wxStaticBox itself and
-    failure to do so will result in warning messages in debug builds. Please see
-    wxStaticBox documentation for more details.
+    failure to do so will result in warning messages in debug builds, even if
+    creating them using the static box parent as parent still works too (but
+    note that items using different parents can't be used inside the same
+    sizer, i.e. all of them should be children either of the box itself or of
+    its parent and an assert will be triggered if this is not the case).
+    Please see wxStaticBox documentation for more details.
 
     Example of use of this class:
     @code
diff --git a/src/common/sizer.cpp b/src/common/sizer.cpp
index 10ce4641c5..f0a99ff656 100644
--- a/src/common/sizer.cpp
+++ b/src/common/sizer.cpp
@@ -2789,6 +2789,8 @@ wxSizerItem* wxStaticBoxSizer::DoInsert(size_t index, wxSizerItem* item)
         // too for compatibility.
         if ( !m_staticBox->IsDescendant(win) )
         {
+            m_hasNonBoxChildren = true;
+
             wxLogDebug("Element %s of wxStaticBoxSizer should be created "
                        "as child of its wxStaticBox and not of %s.",
                        wxDumpWindow(win),
@@ -2805,6 +2807,30 @@ wxSizerItem* wxStaticBoxSizer::DoInsert(size_t index, wxSizerItem* item)
             m_staticBox->MSWDisableComposited();
 #endif // __WXMSW__
         }
+
+        // Also check for the non-supported scenario in which both windows
+        // using box as their parent and the ones using its parent are used:
+        // this won't work correctly because the latter ones won't use the
+        // required offset and so won't be laid out correctly, see the code
+        // dealing with m_position in RepositionChildren() below.
+        if ( m_hasNonBoxChildren && !m_staticBox->GetChildren().empty() )
+        {
+            wxASSERT_MSG
+            (
+                !m_hasNonBoxChildren,
+                "All items of wxStaticBoxSizer must use the same parent.\n"
+                "\n"
+                "It is strongly recommended to use the associated static box "
+                "as the parent for all of them, but if not, all items must "
+                "use the containing window as parent.\n"
+                "\n"
+                "Mixing items using different parents inside the same sizer "
+                "is NOT supported."
+            );
+
+            // One assert is enough, so reset it to avoid giving it again.
+            m_hasNonBoxChildren = false;
+        }
     }
 
     return wxBoxSizer::DoInsert(index, item);