When two fence posts (regular, or Nether Brick) are connected together, you can walk through the middle of them as if the bars connecting the fences have no collision.

When I walked into the middle of the fence, I expected my character to stop, like he was walking into any other solid block.

What actually happened was, I was able to walk through the middle of the fence. You can also jump over the middle.

Steps to Reproduce:
1. Place down two (or more) regular or Nether Brick fences next to each other.
2. Walk into/jump over the middle of the fence - where the two connect.