Who’s At Fault, Developer or Tester?

I don't believe that to be a true statement in general, but I would claim it about a system above a certain level of complexity without a formal proof, or tests covering every possible input(which is usually unfeasible.)

Every software project worth building is above that level of complexity, and it typically doesn't have that formal proof or test suite. Therefore I would claim every system in the wild is more than likely not bug free(or at least not proven to be ). Your train ride home should be safe though because (at least when I worked in industrial automation) there are rules against software in safety devices.