A rather old wiki was overloading their cache with some weird category issues unique to the wiki, causing their cluster to crash, which brought down parts of the platform.
That happened early morning on October 28th (US time) before the wiki migrated to the UCP and then again October 29th in the early morning (US time). We temporarily disabled the category feature that was affected on that wiki specifically and changed its cache to temporarily resolve the issue.
A thorough investigation as to the systemic cause and a resolution for it is underway with high priority. Once that fix is deployed, this wiki will never cause this issue again.
We will not be providing any details on the wiki which caused the issue or details of the cause until the fix is applied.