Bit of a Blooper?
First off, I have to say that I really enjoyed this movie. Whether Peck's Hornblower was true to the literary character is beside the point for me. I just found this to be a entirely enjoyable movie.
And about the blooper business. After Hornblower, Quist, and Bush board the Witch of Endor they encourage the British prisoners to take over the ship. In the ensuing fight (check the guys in the background carefully - IIRC the "bloop" takes place center left)lots of wild swinging takes place. But it looks to me like one of the extras really takes an elbow to the mouth. Check it out and see what you think.