Incorrectly debiting my account

I have taken out a loan with FNB on the 26th May. My loan repayment debit order is captured for the last day of the month as my salary date is the 30th however FNB debits me on 20/06 which is 10 days earlier! I called them and the agent told me they don't know why the debit order went off early but the consultant assured me it won't happen again. This month the debit order went off again on the 20th! What is up with FNB doing what they want without confirming with the customer? I will await feedback