Mirror Causes Left Handed Local Transform Notice how the object local transform axes are left handed after the mirror.  Is this a problem?  It's a little odd for me from a python scripting, I often transform objects with python matrices.  Is it just that the word matrix represents a left handed set of axes?  

So to test, I use the world matrix on each of these objects to inspect what Blender thinks is the positive local x axis.  Strangely, I get 2 different answers depending on how I ask what the local x axis is!  

import bpy
from mathutils import Vector

X = Vector((1,0,0))
for ob in bpy.data.objects:
    mx = ob.matrix_world
    mxrot = mx.to_3x3()
    quat = mx.to_quaternion()

    print(mxrot*X)
    print(quat*X)


result:
Vector (1.0000, 0.0000, 0.0000)

Vector (1.0000, 0.0000, 0.0000)

Vector (-1.0000, 0.0000, 0.0000)

Vector (1.0000, 0.0000, 0.0000)
Turns out  the "problem" is that the scale was -1 after the mirror operator.   After applying scale, the coordinate system was right handed again.

-P