Standard Bank
Running a debit order before the agreement date

I have a revolving loan with standard bank which I am paying via debit order on my salary date, the 20th of every month. On the 18/10/13 they ran a debit on my account which obviously was returned because there was no funds. They do that again on the 19th. What gives standard bank the right to debit before the agreed date? Who is responsible for paying unpaid debit order fees? What about my credit record which gets tainted because it shows unpaid items on my statement. Should I be worried now everytime the 20th falls on the weekend because standard bank cannot stick to its own contractual terms? Rather cancel the debit order I will pay manually!!!