Apply by doing:
	cd /usr/src
	patch -p0 < 001_pflow.patch

Then build and install a new kernel.

Index: sys/net/if_pflow.c
===================================================================
RCS file: /vol/openbsd/cvs/src/sys/net/if_pflow.c,v
retrieving revision 1.32
diff -u -p -r1.32 if_pflow.c
--- sys/net/if_pflow.c	5 Jul 2013 17:14:27 -0000	1.32
+++ sys/net/if_pflow.c	7 Nov 2013 16:48:45 -0000
@@ -269,6 +269,12 @@ pflow_clone_destroy(struct ifnet *ifp)
 	int			 s;
 
 	s = splnet();
+	if (timeout_initialized(&sc->sc_tmo))
+		timeout_del(&sc->sc_tmo);
+	if (timeout_initialized(&sc->sc_tmo6))
+		timeout_del(&sc->sc_tmo6);
+	if (timeout_initialized(&sc->sc_tmo_tmpl))
+		timeout_del(&sc->sc_tmo_tmpl);
 	pflow_flush(sc);
 	if_detach(ifp);
 	SLIST_REMOVE(&pflowif_list, sc, pflow_softc, sc_next);