I changed the type of my banking account, and this one required a higher minimum balance than my previous account, in order not to be charged the "dreaded" monthly fee. I don't like paying banking monthly fees and I think only once did I pay one when they increased the minimum balance and I failed to notice. Never again I said back then :).
So I made the change in the middle of the month and I had a feeling this would happen: the bank charged me the fee as I hadn't had in my account the minimum balance every day of the month.
I'd called them and they said they would make an "exception" - as if it wasn't their mistake - and reverse the fee.
Their system is not set-up properly, the minimum balance should have been considered from the day I changed my account type not from the beginning of the month.