Block Duplication Bug with Pistons (ghost blocks)

Two things: A) The "duplicated" blocks are just ghost blocks, try rightclicking.
B) Exploits like this have previously always been marked as private (if I'm not mistaken, correct if I'm wrong), so if you do find an actual dupe, mark it as "Private" please.

I have the same problem with sticky pistons and redstone blocks in pre 1 and 2. My witch farm produced this bank of ghost redstone blocks: https://imgur.com/miUlqwe

Happened to me yesterday on Pre-Release 1 & 2 with Diamond Block, Cobblestone and Granite yesterday. This bug seems related to every block pushed by a Sticky Piston no matter how much Redstone tick pulse. I built another Iron Farm on my mew world and the farm keep resetting over and over because of this bug. So I loaded my new World in 1.8.9 and my farm is working like a charm without these duplicated blocks.

I think I found a way to trigger this bug consistently in 1.9 pre 2:

Place a sticky piston, attach a block on it and set up a redstone clock to repeatedly extend and retract the piston. The period of the clock is irrelevant. Then walk away backwards while watching the piston. When you get far enough, the block gets duplicated. This happens exactly at the 64 block euclidean (spherical) distance.

When I'm outside that 64 block sphere, the block duplicates every time the piston retracts, but the duplicate block won't get pushed further when the piston extends again.

When I'm inside the sphere, the piston won't duplicate the block when it retracts, but when it extends, it will push the ghost block further if one exists on the way of the real block the piston is pushing.

So if I repeatedly step outside the sphere when the piston is extended and inside when it is retracted, I can make the contraption generate 12 block column of ghost blocks.

Video: https://www.youtube.com/watch?v=ugWqDLBZvEo

As these are just ghostblocks and not real blocks, I'd rather keep this bug, as - in case they're still working like before - they could be a sophisticated mapmaking tool };]
That being said, I know @unknown wanted to work on some piston modification suggestions, to make pistons better, not so buggy, I've got no idea in what way it'd affect bugs like this.
Hope he can say a word or two regarding that 😸
He knows Slime- and Ghostblocks and their potential for the tech community best I guess.

As stated by Meri Diana "these are just ghostblocks..." however they do block player movement, you can stand on them and cannot walk through them, while mobs, water, mine carts, items, etc can.

Related? :
Items such as a that in a water stream from a farm, while not restricted in movement seem to glitch erratically and loss of items is the result. This item movement behavior seems to also be related to the unloading / loading of chunks.

@unknown I don't see much of a problem, if, as you stated, it got no implications whatsoever for mobs, water, minecarts, items etc., but is solely on the player's/client-side.
Not only can you do fun stuff with such ghostblocks like e.g. a ghostblock elevator: https://www.youtube.com/watch?v=sFAlX7TKz1s
or a ghostblock drawbridge where only you, but not hostiles can walk: https://www.youtube.com/watch?v=Og6bdoy92o0

In general, the Survival tech community embraces this bug for their contraptions, and it is not game-breaking, so, personally, and in favour of the tech community, I don't see any issues, even more so as ghostblocks are only a temporary thing and vanish or can be eliminated easily };]
I can also see good use for sophisticated, mind-boggling map technology.

I'm not sure about loss of items in waterstreams nor how ghostblocks affect loading/unloading of chunks, maybe @unknown can help here.
It could be a really good argument in removing them, if it affects the gameplay negatively in any way, but I don't know enough about them to confirm this.

I'm aware that bugs need to be fixed, but I'd rather see the priority with really game-breaking bugs, for the Survival as well as Creative community, and let the tech community have fun with bugs that are rather "good bugs".

@unknown Sorry for falling into your back there Meri but I don't think the argument should be about whether ghost blocks need fixing or not.
While it's in some cases fun to play with them, they are still super derpy and unreliable behaviour that should be fixed eventually.

@unknown Items shouldn't be affected by ghost blocks. At least not really, but just client side. It might look like the item disappeared but after reconnecting it should be in the right position again. The loss of items is almost certainly unrelated to ghost blocks.

Ghost blocks are blocks that just exist on the client side of the game, but not for the server. You can't mine them to get the block back and they will disappear when relogging or when updating them (e.g. right clicking).
The title should probably be updated as this is by far not as severe as duplicating blocks 🙂

@unknown Don't worry about "falling into my back", I trust your word 😸
That being said, of course I also say all bugs need to be fixed at some point, I just feel this one doesn't have to as much as others have to atm ;-;

So as it's client-side we can deduct what @unknown said about having effect on the chunk loading is not correct?
That would have been the sole argument for me to sort of priotize this bug here more.

• And, yes, the title (and description "You probably don't want this to be exploited to the extreme. Right?") should definitely be updated 😸

@unknown 🙂
I agree with that.

What @unknown said is most likely unrelated to ghost blocks, unless it only happens in single player and would be caused by some cross connection between the client and the internal server. But I don't think that is the case.
Is there a bug post about what he described?

@unknown to clarify my original comment, I never stated that items were effecting chunk loading, but the other way around 🙂

@unknown the loss of items I experienced may, or may not be related to ghost blocks. I thought it worth mentioning here since this loss of items has happened to me several times in 1.9-pre2 and each time there were ghost blocks nearby. There is more to that story but this is not bug report to post it in. This has been difficult to recreate and never happens when I need it to. So to stay on topic, chunk loading seems to be causing ghost blocks, and dropped items behavior as well. In the pictures you will see I was testing if ghost blocks can effect item movement. This took a total of 3 minutes. I used 3 wool items in a water stream: 1 yellow, 1 red, and 1 blue. I moved away from the test area by 100 to 150 meters and returned. I did this several times and each time took a screen shot. I noticed I did not see the blue wool anywhere in the test area. I dropped into the water stream to pick them up but the blue wool item never came into my inventory. After I restarted the world there still was no blue wool item anywhere.

5 Screen shots:
http://imgur.com/a/UGHNd

The problem with these ghost blocks seem to be that they cause quite a bit of lag. Slime block mob swipers reduce FPS quite drastically now, no comparision to what they did before this bug was introduced. It makes them almost unusable. If it wasn't for the lag, this bug wasn't so bad, yet still somewhat annoying.

I'm quite disappointed that 1.9 was released without fixing this bug. It has been a real showstopper for me and currently prevents me from upgrading my survival world to 1.9.

It is not just a cosmetic problem. Although the blocks are just client side ghost blocks, they are very real to the player, so the player can't pass through them. Hitting a ghost wall while flying with elytra will stop you, but does not cause the damage that hitting a server side wall would. However hitting a top side of a ghost block after a fall will damage or even kill you even when there was a safe place to land below it. Having your head inside a solid ghost block does not cause suffocation damage.

At least for me this is not a rare glitch. It happens consistently to every piston contraption that is moving while I'm not in immediate vicinity. If the contraption changed state in a way that should create an air block in a space that wasn't air before while I'm more than 64 blocks away but close enough for the contraption to be loaded, there will be a guaranteed ghost block awaiting me when I return.

This simply cannot be described as a "rare glitch", it happens 100% of the time under 1.9. All you need to do is move enough distance away from the contraption and then return to see duplicated blocks.

Please see MC-98410 for another simple example of this bug. This example is very easy to reproduce, and should only take a few minutes to verify.

Myren has a nice video demonstration of the reproducibility of this issue: https://www.youtube.com/watch?v=ox3lAl2rXpM

My opinion would be to remove this bug and turn it into a feature that can be turned on through game settings. These ghost blocks are awesome and would be cool.

This issue is listed as fixed by version 1.9.1 released March 30, 2016. However, I am still experiencing it in version 1.9.4. I've built an elevator and while it will work to move up, when going down, these ghost blocks leave me stuck in the middle of an elevator shaft. Removal of ghost blocks leads to falling (usually to my death).

You are likely experiencing another type of ghost blocks. This ticket is about those introduced in 16w04a.