diff --git a/language/operators.xml b/language/operators.xml
index 2bab4640b2..13d7bfc31c 100644
--- a/language/operators.xml
+++ b/language/operators.xml
@@ -221,6 +221,10 @@ $a = true ? 0 : true ? 1 : 2; // (true ? 0 : true) ? 1 : 2 = 2
 $a = 1;
 $b = 2;
 $a = $b += 3; // $a = ($b += 3) -> $a = 5, $b = 5
+
+// mixing ++ and + produces undefined behavior
+$a = 1;
+echo ++$a + $a++; // may print 4 or 5
 ?>
 ]]>
      </programlisting>