diff --git a/sys/sys/fcntl.h b/sys/sys/fcntl.h
--- a/sys/sys/fcntl.h
+++ b/sys/sys/fcntl.h
@@ -143,6 +143,16 @@
 #define	O_NAMEDATTR	0x04000000	/* NFSv4 named attributes */
 #endif
 
+/*
+ * !!! DANGER !!!
+ *
+ * There are very few bits left for O_* flags.  Every bit we consume for
+ * local features is one bit we can't use for future source compatability
+ * with other operating systems.
+ *
+ * All additions should be coordinated with srcmgr@.
+ */
+
 /*
  * XXX missing O_RSYNC.
  */