The Main Intelligence Directorate of Ukraine's Ministry of Defense claims that pro-Ukrainian hacktivists breached the Russian Center for Space Hydrometeorology, aka "planeta" (планета), and wiped 2 petabytes of data.
Even worse, they are often a case of accretion by generations of grad students and undergrads.
E.g. a university was redoing how it hosted student club websites. When it eventually killed the old hosting, 1 site stayed working. It was eventually tracked down to a little mini pc mounted above the false ceiling. It had been plodding away for 20 odd years, most of that without any maintenance at all.