May 2013 DB server downtime
OpenStreetMap had a very short period of read-only on 25 May 2012. This was to fix an urgent issue with the primary database server indexes.
The database was taken offline (not just read-only)
Initial ETA was "hours", but the sysadmins managed to bring it all back again much quicker (approx 15 minutes outage).
Original mailing list announcment http://lists.openstreetmap.org/pipermail/talk/2013-May/067190.html