diff --git a/gtk/gtkgrid.c b/gtk/gtkgrid.c
index 8a96e82751..03a0976e66 100644
--- a/gtk/gtkgrid.c
+++ b/gtk/gtkgrid.c
@@ -42,7 +42,8 @@
  *
  * Children are added using gtk_grid_attach(). They can span multiple
  * rows or columns. It is also possible to add a child next to an
- * existing child, using gtk_grid_attach_next_to().
+ * existing child, using gtk_grid_attach_next_to(). The behaviour of
+ * GtkGrid when several children occupy the same grid cell is undefined.
  *
  * GtkGrid can be used like a #GtkBox by just using gtk_container_add(),
  * which will place children next to each other in the direction determined