Upon further investigation it seems that during the duel scene, when the gun was fired, pieces of the firearm shattered and one fragment was thrust into actor David Birrell's eye, slicing it and causing heavy bleeding. He shouted in pain and was taken offstage.
I also read that the show CONTINUED AND CONCLUDED. This was during a matinee, so the evening performance was canceled. Monday and Wednesday's performances have been canceled and they are hoping to resume on Thursday (at the Donmar they do not use understudies).
What a horrible incident! I do hope his injury does not permanently affect his vision! Thoughts and prayers to him, his family, and the theater.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle