The scam whereby banks are permitted to lend out money and count the debt as an asset equivalent to cash on hand. This is done under the assumption that by the time the depositors want their money back, the debtors will have paid their loans back with interest enough to cover the difference.

This, along with fiat money, contributes to inflation.

When depositors want to withdraw more of their money than the bank actually has, you get a run on the bank.