References used to create a features (extrude, weld, hole, and others) that are later "removed" by another feature, like a cut (machining process), cause regeneration failures because of missing references. This is a common practice when representing the machining features in the assembly's tree. The ideal, would be to save/maintain the references or do the regeneration starting from top to bottom of the model tree, since the references still exist when the feature referenced is created.
