Black Rock City
Black Rock City exists one week per year, created as the urban core of the Burning Man festival. For general info on the festival see wikipedia:en:Burning Man.
On alternate years, the city centre approximately moves to one spot or another. For a long time we had data in OSM aroundfor the city as of 2007 and 2008 (Now deleted).
This is "temporary" mapping. Properly, it's ended - and for usual OSM objects that means they get deleted (marked as deleted in the database).
For a while we resisted following this rule because the location of Burning Man is so remote, and the area itself is an empty desert 51 weeks per year (well more like 9 months, with set up and take down), and the spot is just so associated with Burning Man ... and the map is so awesome. No one was concerned about this exceptional, edge case. Key:start_date and Key:end_date were applied (at least to some (all?) objects).
...However it seems to have been deleted now