2 Bugs:

Bug 1: There is a Line on the Base-Part of a Mechanism Editor that is on the Front-Plane

Add Plane to the Line

But cancel the Add Plane dialog (x)

It leaves a rogue Plane in the graphic-area, that is offset from the Front Plane.

It cannot be selected for anything. It is not in the Assembly Tree. It will clear from the graphic area if you try to Add a Mechanism to it but then cancel that command.

Bug 2: There is a Line is on the Base-Part of a Mechanism Editor on the Front-Plane

Add a Plane to the Line with XYZ rotations of 0,90,0 - (e.g.?)

Then go to add a Mechanism to that Plane,

You cannot select the new Plane in the graphic area, but you can in the Assembly Tree.

I think I corrected this ages ago