"Stack Overflow works best with JavaScript enabled", but I have JavaScript enabled by default

This could be caused by Google's CDN being blocked, and hence its jQuery not being available:

<script type="text/javascript"
  src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js">

For some time, the JavaScript also outputted “Stack Overflow works best with JavaScript enabled” when jQuery was not loaded. In March 2011 it became more specific:

// display a warning (like the noscript warning) even if JS is enabled, but
// jQuery isn't there (google CDN blocked etc.)
// -- no jQuery is as good as no JavaScript at all for our purposes
var checkForJQuery = function (siteName) {
  if (!window.jQuery) {
     ...
     warning.innerHTML = siteName + " works best with jQuery enabled";

And somewhere in April 2011 this changed to:

if(!window.jQuery)
  ...
  c.innerHTML = a +
  " requires external JavaScript from another domain, which is blocked or failed to load.";

Note that the above will not work if Stack Exchange's own sstatic.net CDN is blocked, as then the above JavaScript is not even loaded. Except on Meta, where the static resources are not loaded from that domain. So, here on Meta one might get the warning, while on the other sites things fail silently if sstatic.net is blocked.

So, for future searches:

Stack Overflow works best with jQuery enabled
Stack Overflow requires external JavaScript from another domain, which is blocked or failed to load.