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.
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Please attach screenshots.

Those villagers seem to be taking full-body block immersion classes. Seriously though, that's not the way the blocks are supposed to work. At least, I don't think that's how they're supposed to work...

It's funny, I couldn't replicate it, but while I was trying to replicate it, I came across another bug involving the slime blocks and pistons.
It seems that entities go through them if the pistons are continuously moving.
And this time I managed to remember to take screenshots.

Do a search before posting. This is a very old popular bug.
Duplicate of MC-119
This is not a duplicate of MC-119. MC-119 is about mobs glitching into the floor.
This one is about entities walking into ghost blocks.
By ghost block can appear when you mess around with slime blocks. The client might think the block is in one location while the server has it in an other.
There is also a post about ghost blocks already. MC-631 it is.
Can someone relink it?
@qmagnet You could try to phrase the this-is-duplicate-message a little nicer. It's not a bad report and the search function isn't that easy to use. It sucks if you take the time to create an issue and then just get the feeling that you did something wrong. Please consider that 🙂