Freeriding
Buying a security in a cash account and selling it before paying for the purchase — riding free on the proceeds. The penalty: the account is frozen for 90 days, requiring cash up front for every buy.
Freeriding violates Regulation T, which governs the extension of credit — paying for stock with the proceeds of its own sale is credit the cash account does not allow. Day trading in a cash account is the everyday way customers stumble into it.