Confirmed that this is fixed. Just for the sake of thoroughness, I tested this out on the following and you can not place a mirror on top of a mirror on any of the following:
Wall, hard wall, yellow, red, black, and blue gates(and sister gates), all pressure plate types, trapdoors (over both), platforms (over both), bridges, thin ice, fire and spike traps, hot tiles, fluff vents, all kinds of floor (including image).
You can not place a mirror at all on water, shallow water, pits, hold complete and master walls, or (interestingly) crumbly walls and secret walls. However, you can place a build marker for a mirror on crumbly walls, secret walls, pits, water, and shallow water and an engineer will build it. A mirror on a pit, shallow water, or water, can be pushed off onto normal ground once it has been built. This seems a bit buggy, but I'm not sure if it is an entirely separate issue or related to the original mirror bug in this thread. This is all in build 5.0.2.1004
____________________________
Links to neat forum tools that I always have trouble finding:
Click here to view the secret text
[Last edited by Insoluble at 08-03-2015 07:57 AM : specificity]