Well.... while an asteroid annihilating life on the planet isn't likely, the size of the web isn't a joke either, so such a collision is more probable than in other cases...

I suppose this could be made a problem for the user, i.e. if you want to make sure that your page isn't colliding with any other, then simply check first. (Of course there is still a race condition possibility of two colliding addresses being created at around the same time...)