Fences will not join and so allow things to pass through on occasion.

I expected cobblestone fences and wood ones to join to form a contiguous fence like they do when you place a fence gate between them.

What actually happened was that the fences did not join

Steps to Reproduce:
1. Start a new world in creative mode or use an existing one.
2. Get wood fences and cobblestone fences and place one next to the other