Ville Saari
Assigned
Reported
Comments
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.
I observed this bug in 16w06a too. It can happen right under my eyes, no need to go x blocks away. When a sticky piston retracts, it pulls the real block back, but leaves a client side ghost block behind. When the piston extends again, this ghost can get get pushed further and pushes whatever is on its way too. But all this happens in client side only, so the mechanism doesn't really get messed up.
When this happens to a slime block contraption moved by a single piston, then every moving block that should leave empty block behind gets duplicated. It seems to never happen to just a subset of those blocks.
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.