full table leaks happen ALL the time. The reasons you dont notice or hear about it is: - the providers which do this by accident are too small (multiple asn hops away from a major transit provider) to become the best choice for most people - the small guy who does leak the full table to a major transit provider, is adequately filtered by the major transit provider by default - the small guy who leaks to the medium transit provider might take an outage, but may not leak to his upstreams due to outbound filtering or the upstreams filtering

you would be surprised, BGP is an old protocol, has had very little serious security improvements. It currently works more or less based upon the goodwill and discipline of network engineers around the world, because if they screw it up, they usually end up offline and out of a job.