Blocks that have alternating models applied change their models once pretty randomly when a block update happens nearby.
A block does not need to get updated in order to change the model, it is enough if the block update happens near the block.
Normally I can just remove and replace a block and it will have the same model as before. That the model changes however, is not intended. Also this can only happen once per block and then it will stay unless you reload the world or something. It does not happen to all blocks, though and you might have to test several times to see it.

I've created a custom resource pack to make it more visible. The sandstone block alternates between 3 textures there. I choose lurid colors there in order to make it more visible when it changes.
The resource pack is attached for own experiments (I however do not recommend to use it for something else and I am not responsible for any epileptic seizures ). If you create a flat world with the preset "redstone ready", you can test it easily

A gif to show the bug in action:
http://i.imgur.com/laAALsy.gif