Okay. So I was playing Minecraft (like usual). Everything was fine. I was just replacing grass blocks with mycelium blocks in creative mode. I was (by default) left-clicking and right-clicking and then boom! That's what happened. The game thought there was still a grass block there, even though I've punched it. Here, try it yourself, but you have to do it very quick, though. (It does the same with other blocks by the way.) I'm not making it up. It's true. It's ALL true!