Apologies reporting this here; I'm not sure where I should report bugs in ControlTower. I believe there's a bug in ControlTower around line 34: def open @status = :open while (@status == :open) connection = @socket.accept # << here @request_queue.async(@request_group) do The "connection" local variable is used in the async block below to parse the request and send the response. Unless MacRuby's blocks behave differently than regular Ruby, this variable is shared across all activations of that block. As a result, it's possible that two concurrent requests will end up using each others' connection, and usually just blowing up as a result. The fix I've come up with is to wrap the @request_queue.async call in a tap call: def open @status = :open while (@status == :open) conn = @socket.accept conn.tap do |connection| @request_queue.async(@request_group) do Since async can't accept any explicitly-passed state, this seems like the safest way to ensure the connection reference is not shared by separate invocations. I'm not sure if it's possible in GCD, but having async take an optional argument with explicitly-passed state might also be a good way to fix this. - Charlie