What is there in the Internet to stop me from making a mistake and announcing via BGP an aggregate that is larger than the nets I am in charge of?
In principle there is nothing to stop you. The responsibility falls on both ends of the BGP link – you are responsible to filter what you announce and the receiving end – if it has its act together – filters also what it *thinks* it should be hearing from you so as prevent mistakes on your part. Those sites that do not work with access lists and filters and just readily accept what is sent to them are just waiting for a problem to happen. Filtering can either be done at the IP network level or at the BGP path (BGP orgin AS) level. See question 20 below for details on a tool to assist in filtering.