I'm not sure if you have this in your pipeline anyway, but found those cases worth mentioning here.

Multipolygon relations with only one member

relation({{bbox}})
  [type=multipolygon]
  (if:count_members()==1);
(._;>;);
out meta;

Closed ways that are member of more than one multipolygon relation with role outer

(clarified, that only closed ways that are member in more than one multipolygon with role outer may be suspicious)