diff --git a/infrastructure/mk/bsd.port.mk b/infrastructure/mk/bsd.port.mk
index 245ba8077da..0924d8259f0 100644
--- a/infrastructure/mk/bsd.port.mk
+++ b/infrastructure/mk/bsd.port.mk
@@ -1,6 +1,6 @@
 #-*- mode: Makefile; tab-width: 4; -*-
 # ex:ts=4 sw=4 filetype=make:
-#	$OpenBSD: bsd.port.mk,v 1.1155 2012/01/14 12:22:07 espie Exp $
+#	$OpenBSD: bsd.port.mk,v 1.1156 2012/01/21 14:44:40 espie Exp $
 #	$FreeBSD: bsd.port.mk,v 1.264 1996/12/25 02:27:44 imp Exp $
 #	$NetBSD: bsd.port.mk,v 1.62 1998/04/09 12:47:02 hubertf Exp $
 #
@@ -38,6 +38,13 @@ ERRORS += "Fatal: you're not allowed to override $t"
 .  endif
 .endfor
 
+.for f v in bsd.port.mk _BSD_PORT_MK bsd.port.subdir.mk _BSD_PORT_SUBDIR_MK
+.  if defined($v)
+ERRORS += "Fatal: inclusion of bsd.port.mk from $f"
+.  endif
+.endfor
+
+_BSD_PORT_MK = Done
 
 # The definitive source of documentation to this file's user-visible parts
 # is bsd.port.mk(5).
diff --git a/infrastructure/mk/bsd.port.subdir.mk b/infrastructure/mk/bsd.port.subdir.mk
index 09d7b7db400..e891dd143d3 100644
--- a/infrastructure/mk/bsd.port.subdir.mk
+++ b/infrastructure/mk/bsd.port.subdir.mk
@@ -1,7 +1,7 @@
 #-*- mode: Makefile; tab-width: 4; -*-
 # ex:ts=4 sw=4 filetype=make:
 #	from: @(#)bsd.subdir.mk	5.9 (Berkeley) 2/1/91
-#	$OpenBSD: bsd.port.subdir.mk,v 1.103 2011/12/12 14:52:02 espie Exp $
+#	$OpenBSD: bsd.port.subdir.mk,v 1.104 2012/01/21 14:44:40 espie Exp $
 #	FreeBSD Id: bsd.port.subdir.mk,v 1.20 1997/08/22 11:16:15 asami Exp
 #
 # The include file <bsd.port.subdir.mk> contains the default targets
@@ -34,6 +34,21 @@
 #	tags
 #
 
+.if defined(FLAVOR)
+ERRORS += "Fatal: can't flavor a SUBDIR"
+.endif
+.if defined(SUBPACKAGE)
+ERRORS += "Fatal: can't subpackage a SUBDIR"
+.endif
+
+.for f v in bsd.port.mk _BSD_PORT_MK bsd.port.subdir.mk _BSD_PORT_SUBDIR_MK
+.  if defined($v)
+ERRORS += "Fatal: inclusion of bsd.port.subdir.mk from $f"
+.  endif
+.endfor
+
+_BSD_PORT_SUBDIR_MK = Done
+
 .if defined(verbose-show)
 .MAIN: verbose-show
 .elif defined(show)
@@ -202,5 +217,15 @@ ${READMES_TOP}/${PKGPATH}/README.html:
 		${README} > $@
 	@rm ${TMPDIR}/subdirs
 
+.if defined(ERRORS)
+.BEGIN:
+.  for _m in ${ERRORS}
+	@echo 1>&2 ${_m} "(in ${PKGPATH})"
+.  endfor
+.  if !empty(ERRORS:M"Fatal\:*") || !empty(ERRORS:M'Fatal\:*')
+	@exit 1
+.  endif
+.endif
+
 .PHONY: ${_recursive_targets} \
 	${_recursive_depends_targets} clean readmes