Remember the bit in Chapter 27 where Harry has the same conversation each time with his Obliviated instructor in Occlumency?

Harry was finding himself very disturbed by how reproducible human thoughts were when you reset people back to the same initial conditions and exposed them to the same stimuli. It was dispelling illusions that a good reductionist wasn't supposed to have in the first place.

Well, it turns out that this is actually the case:

My dad takes sleeping pills every night, and never remembers anything that happens after he takes them. He will never admit this, however. The last three times he has called me at night shortly after taking his pills and we've had the exact same conversation wherein he's asked me the exact same questions. Not "how have you been?" questions but "what is X" or "when does X happen?" type questions that, once answered, don't need to be re-asked.

I answer them the same, and they always lead into the exact same followup questions. It's like we're performing a play. Or, rather, I'm performing a play where I know all the lines and he's performing an improv routine where he doesn't know any.

It's kind of funny.

Also, the parent Reddit thread is simply excellent. Trust me, you should read it. (Thank Yvain for the link.)