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
Related issues
Comments

Sorry, I could not find that bug...

No worries.
Duplicate of MC-2938 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.