TARGET BLOCKS ARE CURSED!!! Ghost Target Block.
I was making an arrow launcher and i forgot to break the block on the front of the dispenser and this happened.
The arrows activate the target block on the client but not on the server. Because of the arrows original position was on the obsidian block and it was already in deactivated state(not moving), when I moved it - it teleported back to the main position but activated the top target block on my client.