When deleting posts or discussions from a forum with RSS, no editions are made to the feed. As cron tests whether to refresh forum's RSS feed by checking the most recent post's date/time and comparing it to date/time of RSS XML file, it doesn't refresh the feed when posts/discussions are deleted, so they stay in the feed, causing confusion to users (especially by link to deleted discussions). The solution may be to force refreshing of the feed when user has deleted post or discussion, but not in the post deleting function in the library, because the system might end up refreshing the same feed many times due to recursive deleting of child posts or whole discussion.
The attached patch introduces this solution (applied with -p0).