I've had user's complain that when they changed a user's rights, it didn't stick. I wasn't able to reproduce it, so I didn't think too much of it.
Now, though, I think I've found what they were talking about. The first time I tried to change a user's groups, I got an error 500. Unfortunately, nothing was stored in the log. The second time, the change "didn't stick" but I didn't notice until I saw no change was made in the log at the bottom of the page.
I looked for some indication of a problem and finally found it. The message was "Conflict of user rights changes! Please review and confirm your changes."
The message, though, is not highlighted in any way and is easily missed since there is no other indication of a problem except for the missing log entry.