To be honest, generally nothing. We are not very strict about enforcing bandwidth quotas.
But if you do go over by a lot, though, we'll charge you the overage rate that is set for your account. Click "Data Transfer" in the Portal to see the rate in the "Overage Fee" column. Additionally, we may contact you before hand 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.