This is only my feeling about the matter. I believe that it IS out of order. We have had this happen on occasion, in the form of a final prayer or benediction. My pastor, not wanting to cause a scene, allowed the prayer to finish but asked the minister to please do his preaching on his designated sunday.
I have seen this done after the word has gone forth and the guest speaker was acknowledging the guest one guest was overcome by something and started the service all over. In my opinion it seemed to be an attention grabber look at me and see how powerful I am. It was defintely out of order.