There has been a bit of discussion of derailment and a little editting along with it. We need to work out something that can be considered a little more legitimant regarding derailment. However what is considered legitimant is also debateable. Nonetheless there are better ways of handling derailments then deleting posts and things.
A thread derailing usually means that a discussion, or shall we call it a subdiscussion has happened in a topic that is not directly related to the actual topic itself. These have occured in forums for the longest time, and these have occured in usenet and mailinglists for a long time as well. This derailment is usually based on circumstances of the thread, or it happens more suttle in most cases with the topic slowly leading into another topic.
When a discussion does this I have witnessed one really good way of them sorting themselves out. Basically on a lot of forums and mailinglists, when the thread or discussion leads offtopic, or gets derailed they change the topic heading. For example on a mailinglist a discussion about pizza could lead to a discussion about cheese, and a person will realise this and change the subject line from "pizza" to "cheese (was pizza)", and it spawns another discussion. However this is not a mailinglist, so if a discussion about "Windows" lead to a lot of talk about "Linux" in it, the discussion could be renamed from "Windows" to "Windows and Linux". I think this is sensible, as moderation tactics like deletion and editing should only be used as a worst case scenario. In a worst case scenario a new thread could be started with the original posts moved in that direction.
Make sense?