i was nearing the end of my overdraft. id bought £250 of music stuff with my mastercard. my mastercard was set to 'direct debit' rather than 'minimum'.
so at a set date once a month the mastercard tries to clear the total balance.
it will do this even if there is no money in your account. so the money came out putting me £200 over my overdraft. there were 2 transactions after this (i would have had enough in there to cover it if my mastercard hadnt taken the money out) charged at £25 each transaction. i now owe hsbc £78, if i had rang up mastercard and asked them to charge a minimum each month i wouldnt be in this situation. hope this helps any of you guys