Kerbal Space Program

Update:

Issue still exists in 1.6.1

Ran into this myself with some recent craft building.

To place a part in mirror symmetry, the editor needs to rotate the mirrored part so that its plane of symmetry is the reflection of the part being placed, that reflection being done in the mirror plane of the ((great)grand)parent part. Somehow, the editor needs to know the plane of symmetry of the part being placed.
There is no obvious indication of the symmetry plane in the configuration files. Surface-attachable from the core game have their node_attach point in their plane of symmetry, and the local y-axis in their plane of symmetry.

If we patch the configuration files as attached, so that KSP rotates the models of these parts to put their plane of symmetry in the x-y plane (like wing parts) then the behavior in the editor is much better. With branches of several parts connected by their nodes, however, the editor still fails to make mirror symmetry. Surface attaching works. (Radial symmetry always works.)

The attached patch changes the starting orientation of these parts, so it breaks existing craft files that use them, because the craft file stores the rotation of parts from their starting orientation to that used in the craft.