I have found other sites where the same thing happens. What usually happens is that I find a useful site dealing with (say) MM6, and I want to have a look at the page dealing with (say) MM7. So I delete the "MM6" at the end of the address to go up one level, and I find that access is blocked. What I need to do is change the "MM6" to "MM7" in the address, so I can hop straight across to that page without going through the level above.What can be an absolute pest is where the change in the page address is not obvious. Klar's site has some extremely useful stuff, but the last time I looked (which was some years ago), access to the higher levels in the directory tree were blocked, and it was a real trial to get from one "open" page to another. I've still no real idea of what else there is on that site that I could (theoretically) see - if only I could find it!
This happens on sites in the business world as well. Somebody tries to go up a level to navigate to another page, and finds that the higher level is off-limits. Even worse, sometimes the innocent attempt to do this is seen as an attempt to hack into the site, which can lead to a degree of unpleasantness. It's never happened to me, but I know of people to whom it has. 
