Ruby

Trivial example:

[1] pry(main)> class Test < Proc; end => nil [3] pry(main)> Test.new { puts "foo" }.dup => #<Proc:0x00007f91a78913c8 (pry):3> 

Also fairly trivial workaround:

[4] pry(main)> class Test < Proc [4] pry(main)* def dup [4] pry(main)* self.class.new(&super) [4] pry(main)* end [4] pry(main)* end => :dup [5] pry(main)> Test.new { puts "foo" }.dup => #<Test:0x00007f91b8e73e38 (pry):9> 

Seems like a bug though, Proc#dup is probably calling the ruby-c equivalent of Proc.new rather than self.class.new and bypassing subclassing