Game is auotmatically placing blocks (Nether blocks in this case) on any block I glance at that can have a block placed on it within block placement range.

What I expected to happen was...:
Blocks should not autoplace when I look at a block that can have a block placed on it.

What actually happened was...:
Very occassionally, (many times in a session) blocks start placing themselves from my active inventory slot on other blocks that I am looking at.  I have restarted the game, and the bug comes back shortly after.  I have restarted my computer, with the same results.  

Steps to Reproduce:

1. ...I have a placeable block (in this case, nether blocks) in my active inventory slot.
2. ...I place a block where I want it

3. ...The game begins autoplacing the block wherever I am looking at.

4. ...Going into my inventory appears to temporarily abate this issue, but it starts again when I place a block where I want it.