Between 2019-02-28T14:40 and 2019-02-28T14:45, this exception shows up in production for ~~some load.php requests~~ //all HTTP requests on mw1272//.
I've never seen this error before and it seems rather concerning this can happen.
Seems like the cache somehow disappeared on a pooled production app server. Or perhaps a new branch enabled without the localisation cache being created first (which is a fatal error, given we've configured MW to disallow computing/populating LCStore on-demand.)
**Update**: The time frame correlates with [Server Admin Log](https://wikitech.wikimedia.org/wiki/Server_admin_log/Archive_37) entries:
* 14:41 hashar@deploy1001: Synchronized php: group1 wikis to 1.33.0-wmf.19 (duration: 00m 53s).
* 14:46 hashar@deploy1001: scap sync-l10n completed (1.33.0-wmf.19) (duration: 03m 33s).
* 14:46 hashar: mw1272 had /srv/mediawiki/php-1.33.0-wmf.19/includes/cache/localisation/LocalisationCache.php:475) No localisation cache found for English. Please run maintenance/rebuildLocalisationCache.php.
* 14:47 hashar: mw1272 fixed by running "scap sync-l10n" from deploy host.
What led to this problem, and how can we prevent this in the future?