TL;DR According to data from RIPE, over 40% of computers attached to the Internet have a few seconds of clock drift, which with the right combination of headers, will make an HTTP response unintentionally cacheable. Background Like many parts of the HTTP model, caching has been extended and revised multiple times over the years. The result is a confusing set of response header values, which affect the way that the browser may or may-not cache the response.