I secured Windows NT
I once crashed Windows '95 by starting a small visual basic app,then starting up a second instancee, third, etc, until Redmond decided that 56 running programs was the limit. I then tried to log out to stop all the instances without having to click them all separately. This was enough to crash Windows '95. Later I crashed an old NT server by firing ping packets at it without the customary 1 second delay and in other ways.
More interesting was when I upgraded a PC of the local student union from Windows '98 to NT 4.0. This worked until I looked at the registry settings and noticed that mostly any logged in user could change any setting, so I tried to secure it, but I went a little further than intended. Now nobody had access - even Administrator and System. NT could not boot without registry access. The usual trick of trying to upgrade Windows (to the same version as it was running) also failed without registry access. Only reformatting the disc helped.