As we've discussed a number of times on Slaw, a good many hyperlinks break over time as their targets get moved or taken down. This link rot is particularly challenging in academia and in law, where cited authorities are an important component of one's argument.
In a 16 page document available on SSRN three weeks ago, "Perma: Scoping and Addressing the Problem of Link and Reference Rot in Legal Citations," Harvard professors Jonathan Zittrain and Kendra Albert:
. . . document a serious problem of reference rot: more than 70% of the URLs within the Harvard Law Review
. . . [more]