I suspect it is related to the code to decide the rotation of generic, rotatable textures. I suspect it's also probable we'll see it with dirt textures and some kinds of stone, but I've just rushed to get the most easily reproduced example which is only of Netherack and only in the Nether.

I've used Fraps to record a video demonstration of the effect: https://youtu.be/BozurzcfKiI

It's nearly subliminal (and when unexpected in the dark nether, really gives one the heebeejeebees that the ground is shifting beneath your feet. ;P) so a single snapshot can't really communicate much.

But, that said I forgot to leave F3 on for the video so I'll get a separate snapshot of that detail attached as well.

What I expected to happen was...:
That whatever orientation a texture was set to display at the beginning of a scene (eg: a chunk loading) should remain unchanged for me as an observer (even if other players are offered a different rotation) at least until the chunk unloads, or even culls, and then later has to be displayed again. If the rotation never changes under any circumstances due to clever PRNG math then that is probably the safest choice, but the payload is that It should NOT change while I'm looking at it.

What actually happened was...:
Texture rotation jumps, glitches, and changes while I'm looking straight at it.

Steps to Reproduce:
This is just the EASIEST way:
1. Go to nether. Creative mode is just fine to do this.
2. Find a place where you can easily walk or fly towards or from a feature made out of netherack, bonus points if the walls/ceiling/floor around you are also made of netherack
3. Move forwards and backwards as my video demonstrates to invoke the fast and sometimes challenging to consciously spot glitchy rendering behavior.

I have confirmed that the bug triggers sometimes as you change your distance moving towards and away from blocks, but also as lighting recalculations are done. So another very easy way to trigger it is to either begin or end a complex cascade of light-emitting lava. Every time a new block gets or loses it's lava contents in your cascade, many many textures throughout the vertical chunk will flip and glitch about quite regularly.

Posting or removing a torch can cause it, but only for a very brief moment and not every time. Mining a block can even trigger it with the same proviso, most likely due to changing the flood of nearby lighting sources.

2015-11-24: I have now caught an instance of non-grass dirt glitching the same way in the overworld.

Standard Disclaimer: Yes, I know this report is probably a duplicate but after 5 careful keyword searches and reading through over a thousand matches that I'm definitely not a duplicate of, you can't say I didn't give it the old college try before ressing up a new ticket at least. Merging me into the correct stream will be very much appreciated just please don't be too upset with me.