atoll suggested that this might be scheduled downtime, but usually that comes with a big "you guys aren't going to be able to work on deadline day!" email, so I thought I'd file a bug...
Planned downtime. Notification didn't go to all the channels that needed to know.
May I suggest sending a post to dev-planning in the following format: Planned <service> outage YYYY-MM-DD HH:MM-HH-MM GMT the next time an outage like this is planned? Details hidden in a "Feedback wanted" thread aren't exactly viewed by all.
hg.m.o is back up again, post-downtime. Discussion about notifications for downtimes is being continued through email.