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);