Of course we'll never know for sure, but do historians have some reasonable ideas about what knowledge may have been lost at the Library of Serapeum of Alexandria, when it was destroyed by the Decree of Theodosius in 391?
From the movie "Agora" it is shown that the pagan occupants attempted to save whatever they could, and suggests that there may have been philosophical, mathematical, and astronomical research. However, Wikipedia information suggests that at this time the library may not have had such information:
The Serapeum housed part of the Great Library, but it is not known how many, if any, books were contained in it at the time of destruction. Notably, the passage by Socrates makes no clear reference to a library or its contents, only to religious objects. An earlier text by the historian Ammianus Marcellinus indicates that, whatever books might earlier have been housed at the Serapeum, none was there in the last decade of the 4th century. The pagan author Eunapius of Sardis witnessed the demolition, and though he detested Christians, and was a scholar, his account of the Serapeum's destruction makes no mention of any library.
So, my question is: in this or other destructive events, did the Library of Alexandria house information that was lost, and caused for lack of a better term, a setback for the progress of human knowledge?