The link works; the evidence does not
A URL that still loads is no guarantee that the page behind it still supports what you linked. The request succeeds, the browser renders, and the fact you relied on is gone. Researchers call this content drift: a resource keeps its address while its content changes until it no longer represents what someone cited.
Broken links are honest; drift is not
A dead link announces its failure. A 404 or an expired domain tells you something is missing. Content drift never announces itself. The click succeeds, the page looks normal, and nothing warns you that the sentence you came to check was rewritten a day, a month, or a decade ago.
The stockpile page that changed overnight
One documented example comes from the early pandemic. The Strategic National Stockpile page described the stockpile as supporting “state, local, tribal, and territorial responders.” Archived versions from April 3 and April 4, 2020 show the wording changing immediately after public controversy over how the stockpile was supposed to serve states. A Los Alamos web-preservation report uses the two captures as a concrete example of content drift: the live URL survived while the policy language at that URL changed.
How often it happens
Drift is common. In 2016, Shawn Jones and colleagues published Scholarly Context Adrift, examining web references drawn from millions of scholarly articles. Among references for which a representative archived copy could be compared with the live resource, more than three quarters had changed.
Dated records are the workaround
That is why dated captures and version records matter. The Wayback Machine and the Memento protocol are built around the same basic need: reaching a version of a web resource as it existed at a particular time. Recovery has limits. Most references in that 2016 study had no usable archived copy near the publication date, and an archive only preserves what someone captured, whenever they captured it.
When a URL still works, the instinct is to trust it. The fix is a timestamp: save a copy the day you read it, note the date, and link to the capture alongside the address. Drift is silent by default; the way to hear it is to keep a record of what you saw.