Things are usually not properly tested, you have to deal with a lot of interoperability issues from different vendors which only get played out on carrier network, logic fails by employees/integrators such as assigning an ip addresss in one corner of the nation and re-using it elsewhere inadvertently due to poor IP address managment, errors flooding a system which cause it to self-reboot which then signals to other systems to also self-reboot or simply a core central node such as hlr/hss has failed and proper geo-redundancy was not implemented