gem 'mongoid', '=4.0.2' # same for 4.0.1
require 'mongoid'

Mongoid.configure do |c|
  c.connect_to "test"
end
Mongoid::QueryCache.enabled = true

p "Using Mongoid #{Mongoid::VERSION}"

class MyDocument
  include Mongoid::Document

  field :parent_id, type: String
  field :prop, type: Integer
end

def create_docs
  n = ENV['N'].to_i
  n.times do |i|
    MyDocument.create(parent_id: i % 10)
  end
end

def clean_up
  MyDocument.delete_all
end

def execute
  clean_up
  create_docs

  Mongoid.logger = Logger.new($stdout)
  Mongoid.logger.level = Logger::DEBUG
  Moped.logger = Mongoid.logger

  yield

  clean_up
end

def main
  execute do
    MyDocument.where(:parent_id.gte => 0).each do |doc|
      doc.set(prop: Random.rand(100))
    end
  end
rescue => ex
  p ex.message
  puts ex.backtrace.join("\n")
end

main

~/lab/mongoid-issue$ N=102 ruby mongoid_issue.rb 
"Using Mongoid 4.0.2"
...
# [ lots of MOPED UPDATE commands ]
...
D, [2015-03-03T16:37:32.024320 #17685] DEBUG -- :   MOPED: 127.0.0.1:27017 GET_MORE     database=test collection=my_documents limit=0 cursor_id=856692613554 runtime: 0.8233ms
"undefined method `push' for nil:NilClass"
/home/elod/.rvm/gems/ruby-2.2.0/gems/mongoid-4.0.2/lib/mongoid/query_cache.rb:141:in `with_cache'
/home/elod/.rvm/gems/ruby-2.2.0/gems/mongoid-4.0.2/lib/mongoid/query_cache.rb:238:in `get_more'
/home/elod/.rvm/gems/ruby-2.2.0/gems/moped-2.0.4/lib/moped/cursor.rb:31:in `each'
/home/elod/.rvm/gems/ruby-2.2.0/gems/moped-2.0.4/lib/moped/query.rb:77:in `each'
/home/elod/.rvm/gems/ruby-2.2.0/gems/mongoid-4.0.2/lib/mongoid/contextual/mongo.rb:122:in `each'
/home/elod/.rvm/gems/ruby-2.2.0/gems/mongoid-4.0.2/lib/mongoid/contextual.rb:20:in `each'
mongoid_issue.rb:44:in `block in main'
mongoid_issue.rb:37:in `execute'
mongoid_issue.rb:43:in `main'
mongoid_issue.rb:53:in `<main>'