Invisible, indestructable block created by piston

I was building a piston-based mob farm (in the Nether), and I found that three of the blocks that I had in front of sticky pistons had disappeared. When I investigated more closely, they were still there, only they were invisible and indestructible by any normal means. I think these are block 36, and when I right-click them, they disappear.

I've seen other bugs talking about this (MC-2167), but I think they are talking about blocks in an active circuit. These blocks remain even if the circuit is disabled.

I do not have a recipe for reproducing this.

This is with a mostly vanilla Minecraft, I'm not running any "mods" and the ONLY thing I've changed is the LWJGL library (to v2.9.0 to avoid sticky key problems). I'll try to make a copy of my world downloadable, but it's more than 10M, so I can't just make it a normal attachment.