During Saturday's regularly scheduled upgrade of Kiln On Demand and FogBugz On Demand, Kiln had a longer than expected amount of downtime.
As part of the upgrade process, Kiln was scheduled to rebuild some of its repository search indices. The resulting system load was orders of magnitude larger than initially anticipated, leading to extremely slow access to Mercurial repositories and 500 errors on some Kiln pages. To remedy the problem, the Kiln team throttled the reindexing task, restoring normal Kiln On Demand access by 2 AM EDT Sunday morning.
Long-term, we're working to better simulate production load in our pre-deployment tests so we can predict issues like this. Short-term, we'll continue to deploy in the wee hours.