An online marketplace runs a shared system that flags accounts likely to be abusing the platform (fake reviews, payment fraud, coordinated scams) so they can be reviewed or restricted. It worked well at launch. Over the last year its hit rate has quietly fallen: the bad actors who get through are the ones who learned what the system looks for and changed just enough to slip past, while a growing number of ordinary sellers complain they were wrongly restricted with no clear way to find out why. The team can change what signals the system uses, what happens when an account is flagged, what is logged, and what review or appeal exists around it.

Decision required: Design how this system should work so it keeps catching abuse over time and treats ordinary users fairly.

Success criteria: It remains effective as the people it targets adapt, AND ordinary users are not driven off by wrong restrictions. Failure = the system degrades into either an evadable rubber stamp or a blunt instrument that punishes the innocent.