Craching

Crache - Poorly configured server caching which prevents distribution of updated files such as CSS, images, and javascript. Crached pages display poorly until the browser cache is cleared.

*** This can be a very subtle problem, as developers often clear the browser cache by force of habit. They would not notice the problem. Site visitors that return to your site after updates may be presented with pages that do not display or function properly.

Key resources

(Apache)

http://httpd.apache.org/docs/2.2/caching.html
http://httpd.apache.org/docs/2.2/mod/mod_expires.html
http://httpd.apache.org/docs/2.2/mod/core.html#fileetag
http://httpd.apache.org/docs/2.2/mod/mod_cache.html

(Mozilla Firebug and YSlow)

https://addons.mozilla.org/en-US/firefox/addon/1843
https://addons.mozilla.org/en-US/firefox/addon/5369