diff --git a/man2/setuid.2 b/man2/setuid.2
index b062a8220..9e21bba78 100644
--- a/man2/setuid.2
+++ b/man2/setuid.2
@@ -72,6 +72,13 @@ On success, zero is returned.
 On error, \-1 is returned, and
 .I errno
 is set appropriately.
+
+.IR Note :
+there are cases where
+.BR setuid ()
+can fail even when the caller is UID 0;
+it is a grave security error to omit checking for a faulure return from
+.BR setuid ().
 .SH ERRORS
 .TP
 .B EAGAIN