https://frackinuniverse.miraheze.org has some Scribunto modules (Lua scripts) that use mw.ext.cargo (as described here: https://www.mediawiki.org/wiki/Extension:Cargo/Other_features#Lua_support ), for example https://frackinuniverse.miraheze.org/wiki/Module:AutomaticInfoboxItem
Sometimes the pages that were working fine yesterday get the following error:
Lua error in Module:AutomaticInfoboxItem at line 44: attempt to index upvalue 'cargo' (a nil value).
(the script has local cargo = mw.ext.cargo)
Manually using purge on the page immediately corrects this 1 page, but readers don't know that, they assume that page is unavailable and are complaining "your wiki is broken".
This happens rarely, but the error gets cached, leaving the affected pages broken until purged manually.
My assumption is, as a part of some maintenance procedure, Cargo is being sometimes disabled (in LocalSettings?) for some reason? And any page that renders during this period of time (even if it's just 2-3 seconds) becomes broken. This particular wiki is heavily reliant on Cargo and less than 1% of pages are usable without it.