The Spaniards torching the library of Texcoco in Mexico, which was reportedly filled with codices like the 4 that survived the Conquest. After the massive epidemics of smallpox that decimated Mexican post-Conquest populations, the Library of Texcoco would have been the only source of detailed information for that region of pre-Columbian Mexico.

Can you explain why the Alexandria part is shunned? I am new to this subbreddit.

There's a way in which the "burning of the library of Alexandria" is metonymy for the general loss of a vast swath of material from the Classical corpus. The loss to culture is huge. We have only one complete "set" of tragedy (the Oresteia) where the three plays meant to be seen together are together. We have only one old comedy playwright with wholly extant plays (Aristophanes). When we've found lost or unknown works, it has lead us to reassess history (in particular, the history found there caused scholars to trust some other historians more and some less).

And sure, you can downplay this by saying that we have a lot of synopses and later classical authors with access to lost sources and fragments and on. You can even take it as truism that the best works are the ones that survived, even though that seems pretty callow ("we've got most of I Love Lucy, so, really do we need any other television comedy prior to 1960?"). Trying to measure what might have been different about history (if, for instance, we had a complete Poetics) is totally in the realm of counterfactual. And it's definitely not a matter of technology we lost, which is the typical framing of the Library.

But for those of us who love poetry and theater, it's a great loss, if only because we can't be certain just how great the loss is.

The torching of the library of Louvain in 1914 has stuck in my mind since /u/Bamboozle_ mentioned it nine months ago. Here's a link to one of the worst atrocities in the first year of WWI.