Mobs (not just the farm animals) glitch through everything: fences, walls, slabs, full blocks. This issue has seriously been around forever, so I doubt it will ever be fixed.
I may be wrong, but I think this problem only occurs when Fast graphics is selected. Minecraft is an insane resource-hog so my computer can't play with Fancy graphics, but when I do use Fancy, I don't have this bug.
I've only ever had this happen when using an unbreaking-enchanted pickaxe in multiplayer. So I assume (as others have said) that it's related to these two things.
I don't think that the burden is really on Mojang for glitches involving texture packs. While I do think they should (and probably will) address TP issues, it's hard to call third-party content problems a bug. And since the latest snapshots introduced changes to the way textures work, this might have altered the way low-res TPs should be made.
@PokeCanada: I've seen a 2x2 texture pack that worked perfectly. Enough said.
(Using MC 1.4.7) For me, this bug is half visual and half physical: my FOV toggles between that of walking and that of sprinting, and my forward momentum is somewhere between sneaking and walking. It's very easy to reproduce: 1) log on to a server; 2) try to sprint a few times; 3) rage. In order to remedy the glitch, you have to take your finger off your forward key (typically W), and try sprinting again—usually takes a few tries before you sprint normally.
This is annoying on all servers, not just PvP servers (but, seriously, have fun with UHC). This bug has been around for as long as I can remember, and it is one of the few bugs that really irritates me.
I apologize. I looked through a dozen of the top issues related specifically to item frames, and could not find this issue. I did not think to search for something else.