FNB dropping the BALL - #Item Paid No1 Items On

I am usually an advocate for FNB and I am generally happy with the service. But I am extremely disappointed at a '#Item Paid No1 Items On' transaction on my account. To my surprise my account is at a negative and a total of R240 is charged as per '#Item Paid No1 Items On'. If there is insufficient funds in my account, why do you not decline the transaction(s)? I ensure that I spend within my means and even bank my change a lot...but this is unacceptable and I request assistance because this should not happen!