The rule that NPCs are recycled after twenty lives, quietly unenforced because deeper audits cost too much.
NPCs are supposed to be recycled after twenty lives. In practice they are not: the Showrunners insisted that expensive, deeper-level audits were unnecessary on the grounds that Class A and Class B NPCs get auto-recycled whenever they are killed, so nobody ever checked.
The result is Juice Box, who has been writing down her memories as they return and believes she has lived at least four hundred separate lives. Justice Light believes her without hesitation. The same neglect explains why the Changelings retain memories of past iterations and why their Changeling Library gating has quietly failed — an accumulation of unaudited exceptions that has produced beings vastly older, better informed, and more dangerous than the system assumes.
The unenforced maintenance rule whose failure produced centuries-old NPCs with intact memories.