Using content_for in Rails 3.0.x can result in a bogus deprecation warning ("<% %> style block helpers are deprecated") because of a bug in how content_for works. content_for should only return stored content when no block has been provided, but instead it returns stored content when the block generates a blank buffer and has a nil return value, which is what happens in a situation like this:

<% content_for :foo do %>
  <% if something_falsy %>
    foo
  <% end %>
  <% if something_else_falsy %>
    bar
  <% end %>
<% end %>

Ultimately this is because capture returns nil (vs '') when the resulting buffer is blank?, but that's apparently intended behaviour, so it's better to fix content_for to explicitly check block_given? rather than relying on the truthiness of capture(&block).