When playing in Creative mode in the last couple of snapshots, I had sticky pistons and slime blocks raising and lowering a Redstone Block at a very fast pace. When I finally broke the circuit, I ended up with two Redstone blocks, one right above the other. It seemed that Minecraft didn't know which one was supposed to be the end position for the original block, so it had one in both the raised piston position and the lowered piston position. It didn't crash my game, though, so I suppose that's good.
Okay, since I couldn't replicate the original issue, have a new one involving block clipping and entities.