ruby -w warns about encoding used as RDoc
$ cat b.rb
# Here is how you set encoding in your file:
# # -- coding: UTF-8 --
$ ruby19 -vw b.rb
ruby 1.9.2dev (2009-05-13 trunk 23418) [i386-darwin9.7.0]
ruby19: warning: encoding 'UTF-8' is ignored, valid only in the first line except for shebang line.
Note: lib/erb.rb has two comments like this, on lines 68 and 72