Possibilities for what you may mean here:
1. Not having parts in assemblies grounded, and then after in-place editting the part, it shifting because it isn't properly grounded or constrained.
2. Editting Extrusions in an assembly, and changing the location of faces of the part, which in turn chages the model to fit the constraints.
3. Extrusions on Extrusions in a part file moving when a face changes.
.... Needless to say, unless you have parts properly constrained or grounded, they will move when you change the geometry of faces on them.