It appears on the subscription page (and as "You have N subscriptions with warnings" on the Dashboard) when a delivery date has more included meals validated on it than the day's Total allows – typically after the day's meals per day or days to delivery were reduced, or duplicate selections were saved. The warning lists each affected date with how many meals are over; click a date to jump to it and use Refund on the extra meals until Selected no longer exceeds Total, which removes the warning.