To be honest, generally nothing. We are not very strict about enforcing bandwidth quotas.
But if you do go over by a lot, we’ll charge you the overage rate that is set for your account; it shows up as a line item on your invoice under Billing > Invoices. Before that happens, we may reach out to discuss a possible account upgrade, to accommodate your needs better, so that there aren’t future overage charges. We’ll even forgive the current charge, within reason.