I've noted that after the title of the question is edited, so change the friendly link, but the old friendly link is still valid and results in redirection to the new link.
According to the question Are friendly links to SO questions permanent? friendly links are permantent, however, the question has not mentioned title change.
So, I'd like to know for sure, and precise that question? If the question title changes a few times, will ALL the friendly links related to the question stay valid forever (well, let's say, as long as the question itself exists), or there is some limit or the time the old link is still valid?