Shopify-maintained Ruby gem that caches a lot of boot-time work to make Rails processes start faster. In the Chattermill incident, a seemingly innocuous minor-version upgrade half a year earlier changed the default cache directory; the deployment script kept writing new cache entries without ever clearing the old location, so after six months the cache reached 10–16 GB. Reading such a large cache at worker startup made Phusion Passenger time out, enter deployment-error mode, and stop spawning new workers — surfacing as recurring production 502s every 30 minutes.