I always thought that apple would warn you or stop you if they see that they have no money in your debit card but in my case, they didn't so an accidental purchase of $29.98 worth of In App Purchases have been charged to my account and since I don't have enough money, I have an outstanding bill of the said amount. The question is, what if I don't pay that amount? I'm a highschool student from the Philippines and that is around 1,300 Philippines Pesos which is pretty hard to earn.
Apple does not have a collection agency that will come after you to try and collect the amount owed. It will just sit there with a negative balance but you will not be able to download app updates or use the account for anything other than viewing the store. After a period of about two years, the account will be closed and any past purchases will no longer be able to be used as any new computers won't be able to be authorized to play them because the account is closed. iTunes is a low profit buisiness for Apple so they do not spend money on collection agencies or attorneys.
You can try and open a new account and be more careful with purchases. They may or may not have updated their system however to prevent users from doing this if they have negavtive balanaces on other accounts. They used to allow people to do this which resulted in people haveing hundreds of open accounts with negative balances.
Apple uses a delayed billng system and sometimes purchaes aren't charged against the card until a balance is reached of about $37. If there are no funds on the card then you get the negative balance and there can be no more purchaes or updates made on the account until the balance is paid. Other than the moment a credit card is added to an account, Apple does not perform periodic checks on the card to make sure there are funds, it just assumes there are and with the delayed billing, it's easy to get a negative balance.