As part of a short term experiment, I enabled the “Auto-close topic after” category setting, setting it to 30 days.
I did not realize it at the time, but making that change had the unexpected effect that a setting further down the page was cleared, the “Auto close topic (n) hours after the last reply.. once the topic has been marked as solved”.
When I decided to change it back to the previous behavior, I did not know what it had been set to previously! 
Moin remembered though so I was able to revert without trouble. I also found the setting changes in the staff action log.
Two thoughts come to mind:
- I can envision cases when you want topics in a category to eventually close automatically whether they are solved or not, but then to close sooner when they have been marked solved. So maybe the setting should be left alone?
- If a setting is changed automatically because of a conflict, there should be some indication in the UI that it is about to happen.