This isn't a spoiler in the conventional sense, but my idea might stink, which is my reason for sticking it with other spoiled stuff.  The idea is preceded by a question, though.  So, I'm following a sort of protocol, I think.  Here is the question: do the wards around the domicile of Harry and Mister shed/eject parasites?  I imagine they must, because I have the impression that exceptions to the wards must be made consciously.  Therefore, uninvited fellow traveler's must be shed somewhere near entry (echh!).  It occurred to me also that internal ...um...symbiots would have to be given a pass.  The consequences of doing otherwise would be even more echhy.  Ok, that wasn't the big idea.  Here it is: what if some nefarious personage attempted to exploit Mister's free pass through the wards to infiltrate Harry and Mister's domicile.  Recognizing the stuff I mentioned above, said nefarious personage would probably exploit the internal passage (hair ball, etc.).  Just a thought....
Second thought: if Harry can make a ward that would shed fleas, ticks, etc. at the door from four legged friends, I think he has a money making proposition on his hands.  It might be marketed as being like the those electronic fences, except it "stops the the bugs, not the pet!"  No doubt the federal government would step in and try to stop it.  Never mind...bad idea.