A route leak is often like this: an ISP in Pakistan is ordered to censor YouTube, so they add a route internally to YouTube's IP addresses that passes to their censoring machine, or to nowhere. They accidentally have their edge routers configured to pass this route to all their connected networks instead of keeping it internally to themselves. Some of their peers recognize this as the shortest route to YouTube and install it into their own networks. Others recognize it's not the real YouTube and ignore it. Transit providers check route authorization more thoroughly than peers, so none of them accept it and the route doesn't spread globally.