Because you are so polite..

i_tried_it.txt

chef > var = node.default[:var]
 => #<Chef::Node::Attribute @normal={...}, @current_normal=nil, @default={...}, @current_default={}, @override={...}, @current_override=nil, @automatic={...}, @current_automatic=nil, @current_nesting_level=[:var], @auto_vivifiy_on_read=true, @set_unless_value_present=false, @set_type=:default, @has_been_read=false> 
chef > var[:a] = 'a'
 => "a" 
chef > var[:b][:c] = 'c'
 => "c" 
chef > node.default[:var]
 => #<Chef::Node::Attribute @normal={...}, @current_normal=nil, @default={...}, @current_default={"a"=>"a", "b"=>{"c"=>"c"}}, override{....}, current_overridenil, automatic{....}, current_automaticnil, current_nesting_level[:var], auto_vivifiy_on_readtrue, set_unless_value_presentfalse, set_type:default, has_been_readfalse 
chef > node.default[:var].to_hash
 => {"a"=>"a", "b"=>{"c"=>"c"}} 

My apologies, I did not describe the bug sufficiently accurately:

require 'rubygems'
require 'chef'
require 'pp'

node = Chef::Node.new
default = node.default
nginx = default[:nginx]
nginx[:core][:user] = "www-data"
nginx[:core][:worker_processes] = 10
pp node.to_hash
# {"name"=>nil,
# "nginx"=>{"core"=>{"core"=>{"worker_processes"=>10}, "user"=>"www-data"}},
# "chef_environment"=>"_default",
# "chef_type"=>"node"}

What is this I don't even.