FNB is the worst bank I have ever banked with

Once again im sitting on a minus because FNB's pathetic system take donkey years to process payments, called the call centre and spoke to a gentleman called Innocent who claims that I made purchases with no money in my account, is that even logical, who makes a purchase knowing very well that they dont have money in their account????? Never have I ever had a good experience while banking with FNB its been the worst 4 years, pleas explain to me why im being charged R150 for items that took 4 days to process??