MIRROR SHAPE KEY Issue - version 2.66

hi guys,
the “mirror shape key” is a very useful option but I’m getting some problems with that. When I mirror a shape key, some vertices move correctly but also moves randomly other vertices somewhere else on the mesh.

Is this a bug?
If so, is there some way to select those additional vertices, which I don’t want to move, and reset them to the ‘basis’ position?

Thank you in advance!! Have a nice day!! :slight_smile: