with block.rb

def session_initiated
  set("foo", "bar") {}
  # @queue: [Proc{}]

  set("lol", "cat") {}
  # @queue: [Proc{}]

  read "/foo/bar.mp3" do |result| 
    ..
  end
  # @queue: [Proc{..}, Proc{update_session}]
end

# Now, the block supplied to read() is called at the fourth response.

without block.rb

def session_initiated
  set "foo", "bar"
  # @queue: []

  set "lol", "cat"
  # @queue: []

  read "/foo/bar.mp3" do |result| 
    ..
  end
  # @queue: [Proc{..}, Proc{update_session}]
end

# Now, both set(), set() and read() generates a response, which will
# call queue_pop. So the block supplied to read() will be called after 
# the second response, which is the response to set(). This results in
# the block being called with *no* parameters–and too early–and thus a 
# *foo called with wrong paramenters (0 for 1)* or something like that.
# Makes sense?