diff --git a/KNOWNBUGS.rb b/KNOWNBUGS.rb
index b97a08d928..36d757846c 100644
--- a/KNOWNBUGS.rb
+++ b/KNOWNBUGS.rb
@@ -3,3 +3,8 @@
 # So all tests will cause failure.
 #
 
+assert_equal 'ok', %q{
+  t = Thread.new { system("false") }
+  t.join
+  $? ? :ng : :ok
+}