In your question you ask us to draw diagams
illustrating why it is not safe. Does this
mean that you expect more than one answer or
situation in which it would not be safe?
If so how many?

I'd expect two diagrams like those I gave you. The two diagrams I gave you are related in that they show two different sequences of events that look the same from one of the computer's (A's) point of view. Since the computer can't tell the difference between the two scenarios, it must act the same in both. To handle the two scenarios correctly, however, it would have to act differently.

Tom