WordPress Plugin caching and Browser caching have been well-discussed in the forum. Both can make settings changes you’ve made not show up on the site until you’ve cleared the cache.
But very recently we’ve learned of another kind of caching that can cause similar issues. It turns out that the latest versions of PHP on your host can also be cached. The actual PHP code is cached, and if you update a plugin or theme, it may take a bit of time for PHP to reload the newest version of the files. There is nothing you can do about this. PHP caching is a configuration option of your PHP installation, and definitely provides significant performance improvements. The latency for this caching seems to be a few minutes at the most, but it is real and under certain circumstances, you might experience this kind of caching artifact.