When you are up close to a fence, and place a block on the side that's facing you, it should not let you place a block...
But of course it does. Because this is a bug.
To Do It
1. Place a fence
2. Face any side of it.
3. Place a block on the side that's facing you
It pushes you aside.
If you don't believe me, try it.
It even works on the top of it.