Owing money I am unaware of due to system error.

I was sent money into my account, but on trying to withdraw it I was told I have insufficient funds. Went to a consultant who informed me that my flexi saving acc was mistakenly made my default savings acc which is a system error and so as I was purchasing they were taking the money from birthday which had no money instead of my savings acc which had money. Now they say I owe them. I am so disgusted by this level of unprofessionalism by Capitec.