Windows 10, Java edition 1.12.1
A flying machine involving pistons, observer blocks, and slime blocks are causing any attached block to periodically leave a trail of fake duplicate blocks.
I attached a sandstone block to the slime block of the flying machine and expected it to simply take me up to the obsidian block I used as a stopper. And it does that. But while I was trying to tinker out a good starting mechanism my mouse double clicked and broke the starter block, updating the observer which caused the attached blocks that were acting as my seat to leave a trail of blocks.
In my Redstone world it seemed to make the duplicates easily but in a newly created world, it took longer for the bug to happen.
Steps to Reproduce:
1. Place a disposable block down.
2. Place an observer block, face down, on the disposable block.
3. Place a slime block on top of the observer block.
4. Place a sticky piston, face down, on the slime block
5. Place a sticky piston, face up, on the right face of the slime block.
6. Place a slime block on top of this sticky piston
7. On top the first sticky piston, place an observer block on the latest slime block.
8. Place the desired block to be duplicated on a slime block. The slime block below an observer block will leave a trail while going up, the slime block above an observer block will leave a trail going down.
9. Place an obsidian block somewhere in the air above the second observer block. This will be where it stops.
I found a way to automate it, I'll attach images of how.