Title is self-explanatory, the corners of the 3x3 square atop of huge mushrooms do not generate with the correct data value.
Ignore the fact that the world hadn't fully loaded in the screenshot, the appearance did not change once it did load.
Environment
Operating System: Windows 8.1 (amd64) version 6.3
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Confirmed, also happens with bonemealed red mushrooms. Brown ones seem to be fine.
The incorrect textures are facing east and west, all 4 corner pieces are wrong.