How did Bob know how to pull off the Darkhallow?
At the end of Chapter 34, Harry says "Bob used to be Kemmler's" and "Bob knows everything about the theory that Kemmler did." And then, near the end of Chapter 42, Bob says "It is precisely as the master described." But all the way back in Chapter 3, Harry orders Bob to permanently forget those memories. We later learn that Bob has expelled those memories into a separate SoI.
What's going on here? Did Jim make a mistake and have Bob remember something he couldn't? Did Jim make a mistake creating Evil Bob in Ghost Story? Was Bob pretending to know more than he did?
If it was left at Bob is just awesome and could figure out how to do a Darkhallow, that would have been fine. And that's what I said when my brother asked me this question. Then I reread the book after he asked me, with that question existing in my mind. The real problem with that explanation is Bob's quote from Chapter 42.