I worked for Honeywell, and HI-Multics (CNO) was our software development host computer. I was starting a new project, and we needed better software Configuration Management tools for development. On Multics, rings sounded like an ideal mechanism to enforce rules without worrying about the programmers going around them. I asked CNO how to get permission to run a program I wrote in Ring 3, where forum and mail and that stuff operated. The answer was no, but "All Multics User functions work in Ring 5." I have that EMail (from Dave Bergum) framed on my wall to this date.
All the documentation clearly states that system software has ring brackets of [1,5,5] so that it runs equally in both rings 4 and 5. However, the PL/I compiler creates segments with ring brackets of [4,4,4] by default. If you're in Ring 4, the default, [1,5,5] and [4,4,4] are the same. Between the time HI-Multics was installed ('79) until I started a project where all the users were locked into Ring 5 ('82), it seems the CNO site analysts had run the PL/I compiler. Several times. When they got a bug fix or hot tip from System-M they just pulled the source out of the library and fixed it. This didn't impact the critical hardcore of Multics, just every little tool and handy utility they provided for users.
I found each and every place CNO had fixed a program without resetting the ring brackets correctly. It started out 5 a day, and in 3 months it was down to one a week.
The project ran several baselines and did hundreds of builds from the Multics regulated CM files. The builds were completely automated using the Multics to Level-6 remote connection software. Despite programmer grumbles, the process integrity provided by Multics worked without an error for 4 years.
At the '84 HLSUA meeting I was talking to the Multicians from Phoenix at the Honeywell Employee session and I wanted to talk to them about more reliable ring bracket settings with add-on software. There I learned that I was the only known (at that time) user who was using ring gates of their own design. I was going to write a paper about it for the next HLSUA, but then "Multics Users met their Maker".