At 10:57 UTC, the master database server for s3 (the cluster that holds most of our wikis) had a full disk and stopped writing. For this reason it was no longer possible to edit these wikis. The larger wikis live on separate clusters and were not affected.
After switching to another master database, all wikis are back up and editable as of 12:02 UTC. A few edits that were made during the incident may have been lost.