Sorry having to bring this one up here, but it emerged from a discussion with another mapper (and it seems there's no example for this on the wiki).

This is reported by OSMI check "Way in > 1 rings" as an error. My understanding is that osm multipolygons conform to OGC SF except for touching inner rings, which can be combined to form an SF multipolygon. If that is true, then the SF's assertion of

"The interior of a MultiPolygon with more than 1 Polygon is not connected; the number of connected components of the interior of a MultiPolygon is equal to the number of Polygons in the MultiPolygon."

would rule out both cases above. Then again, not all mappers seem to share this view, and I know there's been a lot of discussion about OGC conformance...