Search engine spiders, scrapers, etc. would end up hitting a bunch of dupes when the title change causes a new URL to be generated that really points to the same content (not that spiders don't already have to deal with that mess on other websites).
HTML headers can include a `<link rel="canonical" …>` that points to the up-to-date URL of this item. Furthermore, outdated or invalid titles could be redirected with a "301 moved permanently" to the current title.