When a fence or wall is placed next to a block so they link then from some angles parts of the linking become transparent. The more blocks linked the worse the rendering (see forums link below for more screen shots).

See also:

https://www.minecraftforum.net/topic/1533766-fence-texturing-broken/

http://www.minecraftwiki.net/wiki/Issues/1.4.1pre#Graphical.2Flighting