<!-- Piece of erb code -->
<% unless_error do %>   # Error: wrong number of arguments (1 for 2)
   <p>Some HTML here</p>
<% end %>


# MyHelper.rb:
def unless_error &block
  if flash[:error]
     concat "<p>#{flash[:error]}</p>"
  else
     # Execute given erb block:
     yield
  end
end

# The problem is the error message on line 2.