class Example
  include Mongoid::Document

  field :test, type: Hash
end

e = Example.create!(test: { "bad.1" => "bad key" })
#=> #<Example _id: 56ce62a86572697d57000000, test: {"bad.1"=>"bad key"}>
e.save!
#=> true
e.test.merge!("bad.2" => "bad key")
e.save
Mongo::Error::OperationFailure: The dotted field 'bad.1' in 'test.bad.1' is not valid for storage. (57)
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongo-2.1.2/lib/mongo/operation/result.rb:226:in `validate!'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongo-2.1.2/lib/mongo/operation/write/write_command_enabled.rb:48:in `execute_write_command'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongo-2.1.2/lib/mongo/operation/write/write_command_enabled.rb:38:in `execute'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongo-2.1.2/lib/mongo/collection/view/writable.rb:193:in `block in update'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongo-2.1.2/lib/mongo/retryable.rb:84:in `call'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongo-2.1.2/lib/mongo/retryable.rb:84:in `write_with_retry'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongo-2.1.2/lib/mongo/collection/view/writable.rb:184:in `update'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongo-2.1.2/lib/mongo/collection/view/writable.rb:167:in `update_one'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongoid-5.0.2/lib/mongoid/query_cache.rb:168:in `update_one_with_clear_cache'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongoid-5.0.2/lib/mongoid/persistable/updatable.rb:144:in `block in update_document'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mongoid-5.0.2/lib/mongoid/persistable/updatable.rb:118:in `block (2 levels) in prepare_update'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-4.2.4/lib/active_support/callbacks.rb:117:in `call'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-4.2.4/lib/active_support/callbacks.rb:117:in `call'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-4.2.4/lib/active_support/callbacks.rb:555:in `block (2 levels) in compile'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-4.2.4/lib/active_support/callbacks.rb:505:in `call'
        from /.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-4.2.4/lib/active_support/callbacks.rb:505:in `call'
... 4 levels...