diff --git a/docs/reference/glib/tmpl/threads.sgml b/docs/reference/glib/tmpl/threads.sgml
index 3ee9d61c3..58c62bdd1 100644
--- a/docs/reference/glib/tmpl/threads.sgml
+++ b/docs/reference/glib/tmpl/threads.sgml
@@ -693,6 +693,12 @@ called, and, in that case, will do nothing.
 Destroys @mutex.
 </para>
 
+<note>
+<para>
+Calling g_mutex_free() on a locked mutex may result in undefined behaviour.
+</para>
+</note>
+
 @mutex: a #GMutex.
 
 
@@ -837,6 +843,12 @@ unbounded lifetime, i.e. objects declared 'static', but if you have a
 you should also free the #GStaticMutex.
 </para>
 
+<note>
+<para>
+Calling g_static_mutex_free() on a locked mutex may result in undefined behaviour.
+</para>
+</note>
+
 @mutex: a #GStaticMutex to be freed.