It's possible to become overdrawn beyond your agreed limit if the final payment amount requested by a merchant exceeds the initial authorisation, or if the transaction was processed offline. In these situations the amount requested must be paid and cannot be declined, even if you have insufficient funds. This can lead to your account becoming overdrawn (e.g. the payment amount exceeds your balance).
Rest assured, Starling does not charge one-time fees or interest in these situations. It is still important that you pay off the amount though to avoid your account going into arrears.