MediaWiki problems but good news for Toolserver stability: Deployments of MediaWiki 1.21wmf5 cause widespread problems for users across wikis when HTML and CSS updates came temporarily out of sync. On the first wikis targeted for deployment, this was caused by the different cache invalidation rates for HTML (typically one month) and CSS (typically five minutes). The retrospective on the problem highlighted the fact that that the test wiki – the WMF's answer to a production environment that individual developers can no longer practically emulate themselves – actually demonstrated the exact problem that would later manifest itself on production wikis. It went unnoticed.
