A user report indicates that when "hide out of stock products from the catalog" is enabled in WooCommerce, and a product goes out of stock, the catalog pages are not updated (i.e., the old cache file is loaded, which shows the now-out-of-stock product that should be hidden).
In #597 we improved compatibility with WooCommerce and stock changes, however it sounds like there's room for more improvement here so I'm opening this GitHub issue so that we can look into this more closely.