When it happens, be up front about it. If Person B tells you something different that someone you spoke to before them, tell that person directly: "I spoke to person A earlier, and they said [whatever it was]; how would that fit with what you told me?" More often that not, Person B will either double-check to make sure they were correct, or admit that they don't know. In most retail situations, this should work well. In government-affiliated offices, this may not work as well, since the bureaucrats working there don't have to worry about customer service.