So I extended a piston with a rail in front of it, and while it was extending, the rail seemed to occupy a full block. The same thing happens when you retract a piston.
After some further testing I found that it happens to all non-full blocks. Fences for example, turn into wood blocks, carpet turns into wool and slabs turn into full blocks. (After the extension and retraction they are turned back to normal though, so it's only visual.)
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Confirmed for 14w17a

Confirmed for 14w18a

I have also seen that blocks' textures get flipped while they're being moved by a piston.

Paul, this has already been reported. See MC-54053

Oh, thanks

Confirmed for 14w19a.

Confirmed for 14w20a and b.

Confirmed for 14w21b
Glass panel and sandstone (type 1; piston facing up)

If you attach a lead to a fence and perform this glitch, it pops off; I think it's more than just a visual issue.
I guess this is a different issue; it still happens (knots are entities). It might even be a feature request...