I've had this recurring problem through several versions of WordPress. It's been the most annoying bug I have encountered thus far, because it goes unnoticed until a reader is kind enough to contact me personally to let me know that it's happened.
The problem is that the discussion / comments settings change by themselves, from my selected preference that users NOT need to be registered and logged in to comment -- to that do DO need to be registered/logged in. That means a lot of folks run into trouble to leave comments, because all of a sudden they need to be registered.
Has anyone experienced this? Any suggestions?