There's some indication that you needed to enable the guest account in Leopard (it's disabled by default) then upgrade to a developer seed, and then to the reta

There's some indication that you needed to enable the guest account in Leopard (it's disabled by default) then upgrade to a developer seed, and then to the retail Snow Leopard. If true, that would explain why is wasn't caught sooner. If true, you could test the final build of Snow Leopard all day (and the upgrade path from Leopard) and not catch it.