Whenever I hear news like this, part of me feels bad for all the programmers working very long hours to fix the problem. You can be sure that a lot of pagers started to go off as soon as the reports of billing issues started to come in.
My partner still gets a pager for on call at a hospital, she gets a phone as well.
A colleague is a former AWS employee - my understanding is nobody wants to touch the billing code for this reason.
I think that also explains why the billing interface leaves a bit to be desired.