It is possible for agreements to be created with out a start date. In this case no payments will occur until you visit WAS and enter a start date.
Subsequent payments happen regularly at an interval that you specify. If you specify a date that does not occur in a particular month (such as 30th of every month, which does not exist in February) then the payment will happen on the last day of the month.
The customer can take various actions to rectify the situation. In case 1 he can change the credit/debit card from which payments are made; in case 2 he can top up his WorldAccount. Both of these can be done on the Web-Merchant/WM Administration Server (WAS).