Any sane bgp neighbor has prefix limit ACLs and prefix filters in place on their edge connections to another peer. As an ISP that announces a lot of space and customer space to 2 upstream transits, every time we take on a new downstream customer that brings their own /24 or bigger, we need to have our upstream transit providers update their prefix-list filters.

If our upstreams were clueless or negligent, it would be possible to get into a situation such as when a Pakistani telecom announced a huge chunk of V4 space that is YouTube, effectively DDoSing their international submarine links and also taking down YouTube for some users worldwide.