Side note: Paper corrected this bug as of release 1168, five days earlier. Spigot server owners should take a serious look at Paper, as bugs like this often are fixed earlier.
Server owners will likely breathe a huge sign of relief with this one. The crux of this update was to correct a serious game breaking bug allowing very easy item duplication. Side note: hopper duplication still persists. This particular bug – a flaw with the new craft book system – was widely disseminated by various YouTubers, here’s Docm on the matter: