Massachusetts Cash WinFall roll-down exploit
When the Cash WinFall jackpot hit its cap without a winner, the prize money rolled down to the lower tiers, which briefly gave each ticket a positive expected value. MIT students and a retired Michigan couple (the Selbees) spotted this and bought hundreds of thousands of tickets during roll-down weeks. The lottery's own rule, meant to spread payouts around, created an edge that anyone who did the arithmetic could exploit.
