Dellavalle’s concerns reflect those of a growing number of scientists and scholars who are nervous about their increasing reliance on a medium that is proving far more ephemeral than archival. In one recent study, one-fifth of the Internet addresses used in a Web-based high school science curriculum disappeared over 12 months. | Another study, published in January, found that 40 percent to 50 percent of the URLs referenced in articles in two computing journals were inaccessible within four years. —Rick Weiss
—Web sites vanish so fast scientific papers just can’t keep up… (SF Gate)

I wonder how many of these broken URLs are caused by the actions of webmasters who temporarily publish articles for free, and then pull them behind pay-per-view walls. A researcher who cites a URL that appears to be free may not know it has an expiry date. FYI, the above article originated from the Washington Post, whose free articles expire after a few weeks.