Hi FDP, it probably means the orientation of the object was not correct - that the geometry library had analyzed the object and got the decision wrong about which side was pointing to the outside of the closed volume... It could be due to a bug in the geometry library or problems with the object itself if you have any surfaces in it which fold back over top of themselves in a self-intersecting manner.

Anyway it's likely to be related to the geometry of the particular objects you were using at that moment.

- Michael