http://puu.sh/qnvRU/be257cf1bb.gif (File mirror {F8858} )
Placing down mirror Axis and attempting to place down the Mushroom block in particular will cause it to be rotated in the other axis.
The same thing applies for grate corners and light bar corners, any other LoD objects seems to mirror fine (with an exception for Mirror Cubes but that affects other blocks too)