In JOSM you'll need to force a complete Validator run – make sure no objects are selected, when doing so, or it won't work.

Or have a look at Geofabrik's OSM Inspector[1] (change View to 'Areas') for a visual analysis of the errors.

In short: In a multipolygon-relation an "outer" object never should touch any other object of that same multipolygon. It's the "empty space" (tagged man_made=yes) between the artwork and the facade that is used incorrectly.

Contrary to KISS, this seems like "something ID would suggest", and I don't think ydrgbjo is aware, that iD doesn't have a clue about valid relations.

[1] http://tools.geofabrik.de/osmi/
