I have made a multi-part object by merging smaller polygons from several sources into a single large polygon. When I try and run SQL overlap queries on it to measure overlap with another layer; or cut it into smaller pieces with the "split" function using another layer as a template, I get the following error: "All or part of Object(s) invalid. Results may be incorrect." Parts of the polygon disappear in the cut version, and I can see from the results of the overlap query that the same areas are not being included in the sums. Presumably there is a mapping error in these sub-polygons, but running checks for errors in both Mapinfo and ArcGIS detects gaps and overlaps elsewhere that don't seem to be causing a problem, but finds nothing wrong with the disappearing bits. If I run the "Clean" function on it, the faulty bits just disappear entirely (as when I cut it). For one of the disappearing polygons, which is, topologically speaking, doughnut shaped, it fills in the hole and deletes the rest, so that may be what is happening to the others but with a hole that is too small to see? Does anyone know what is going on and how to fix it?