The Nether portal's portal animation blocks have strange texture/rendering when looking at it from certain angles, such as from the sides or above. I have found that this happens with any sized Nether portal and only happens when playing Minecraft with graphic settings set to Fast, on Fancy graphic settings this problem does not happen and from what I can recall, this problem didnt happen in the previous snapshot or 1.6.2.

I am not using a texture pack.