The Crash of 2018 refers to the loss of over three thousand pages on the DariaWiki.
The exact causes of the crash remain obscure, though it appears to have been an accidental side-effect of transferring the wiki over to a new server. Whatever the cause, on 11th January 2018 the fandom woke up to find vast swathes of the wiki were missing - ranging from fanfics to minor characters to the one for the TV show itself.
Investigation found that every page created since March 2010 was lost, resulting in a surfeit of fandom info for this era.
An oddity of the crash is that:
a) any page created before that date retained the updates it had received since early 2010
b) any uploaded image remained accessible, even if it was in the 2010-2018 span
(In February 2018, an unknown user began erasing fandom information off pages to leave only canon but this was luckily easy to reverse)
The original admin of the wiki vanished and so any backups that may have existed could not be uploaded.
Work on restoring the DariaWiki to its former glory started in April 2018 and is still ongoing. As of Christmas week 2020, there were around 1,500 fewer pages than the start of 2018 - there's likely fandom lore that may never be recovered.