WordPress appends a query var like ?ver=4.2.1 to CSS/JS URIs when building <link> and <script> tags. When WP’s version changes (eg, from 4.2.1 to 4.2.2), the query var changes too, causing browsers to bypass their cached assets and forcing a fresh version to be requested from the server. This works well for core assets, because they generally don’t change between WP releases. But for assets loaded by plugins and themes – which don’t always provide their own ver string when enqueuing their scripts and styles – it’s not a perfect system, since browser caches will only be busted when WordPress core is updated.

I built a small filter for the CUNY Academic Commons that works around the problem by appending our custom CAC_VERSION string to all ver query vars. This ensures that users will get fresh assets after each CAC release, whether or not that release includes a WP update.

Note that this code is pretty aggressive: it busts all CSS/JS caches on every release. You could use the $handle argument to do something more fine-grained. Note also that this technique doesn’t work equally well for all caching setups; see this post for an alternative strategy, based only on filename. Proceed at your own risk.