When harvesting mushroom blocks from the two different colours of huge mushrooms (red and beige), the resulting identical blocks are not stackable, though they are identical in every other way (same name, same texture).
This is occurring on an un-modded vanilla server running Minecraft 1.5.1 in survival mode.
Here is a video demonstrating the bug: https://www.youtube.com/watch?v=qIG_XEiO0Fc
This is my first bug report, so I hope I have included all of the relevant information that is required. Please let me know if additional details are necessary.