diff --git a/KNOWNBUGS.rb b/KNOWNBUGS.rb
index e85a6522a1..c5ebc97b02 100644
--- a/KNOWNBUGS.rb
+++ b/KNOWNBUGS.rb
@@ -10,6 +10,8 @@ class TC_KnownBugs < Test::Unit::TestCase
   end
 
   def test_block_arg1
+    # &b wrongly captures the upper block such as the one given to
+    # this method, if no block is given on yield.
     just_yield {|&b|
       assert_equal(nil, b)
     }