The bug

Occasionally, tile ticks are not removed successfully from the save file, causing them to linger there eternally. This in turn causes the affected blocks to no longer be able to get ticked.

This is known to affect the following blocks:

• Repeaters
• Observers
• Comparators
• Daylight sensors(?)

... so probably all blocks that can receive tile ticks.

When the TileTick values of the affected chunk are purged via a third-party tool, the affected blocks start working again.

To reproduce

As of right now, it is unknown what causes this issue. There are some worlds however in which this issue occurred.

• https://drive.google.com/open?id=1j0nac0qWlOkMg-DFuhjRgrLlUk_upBJk (from this ticket)
• https://mega.nz/#!gFplBCbL!OLzdTb0sMM2KA9_PS_hrHhE7D9I06bOYuM7m7B8EP1M (from MC-158935)

Original description

I've set up a few flying machines using sticky pistons, observers, and slime blocks for a witch hut farm. It worked for a while, but now suddenly the back observers in all 3 flying machines have ceased to work. This is after an overnight AFK session at the farm. I've attached pictures of one of the observers, without the flying machine. The observer are connected to a redstone line for ease of demonstration of the bug. No matter how I update the observer, the observer will never fire, and the redstone line will never turn on. The observers are all facing the correct direction, and this bug only happens at one certain block location in each flying machine platform. All observers in other positions work fine, it's just these three specific positions that seem to be causing the observers to not fire. No other redstone signal is making its way to the observer.

In addition, I've downloaded the same world that the server is running, and the issue still persists in single player. The issue also persists after restarting the server, and still persists after re-uploading the world to the server. Optimizing the world also does not fix the issue