This small testcase triggers this assertion in gtk+:

#include <gtk/gtk.h>

void main (int argc, char **argv)
{
    GtkWidget *toolbar;
    GtkToolItem *button;

    gtk_init (&argc, &argv);

    toolbar = gtk_toolbar_new ();
    button = gtk_toggle_tool_button_new ();

    gtk_toolbar_insert (GTK_TOOLBAR (toolbar), button, 1);
}


It's coming from:

static gint
logical_to_physical (GtkToolbar *toolbar,
                     gint        logical)
{
  GtkToolbarPrivate *priv = toolbar->priv;
  GList *list;
  gint physical;

  g_assert (logical >= 0);

  physical = 0;
  for (list = priv->content; list; list = list->next)
    {
      ToolbarContent *content = list->data;

      if (!toolbar_content_is_placeholder (content))
        {
          if (logical == 0)
            break;
          logical--;
        }

      physical++;
    }

  g_assert (logical == 0);

  return physical;
}

This function asserts if the toolbar contains less 'logical' elements. The test case is trying to insert an item at position 1 in an empty toolbar, so the assert is triggered. Maybe a g_return_val_if_fail (logical == 0, physical); would be better than the assert?