John Spanos

The world in the link contains a region (r.1.4.mca) that will crash the world. Use the following steps to reproduce and to resolve the crash.  Note, you will lose data by deleting chunks, but at least your world will no longer crash.

CrashWorld

Start the world
Teleport to 679 100 2118 (chunk 42, 132) - Can use command block to tp.
Game will crash

To fix, use an NBT editor like NBTExplorer and delete the 9 chunks surrounding chunk 42,132
41, 131
41, 132
41, 133

42, 131
42, 132 * (problem chunk)
42, 133

43, 131
43, 132
43, 133

Start the world again and the chunks will be re-created and no crash.

Just deleting the problem chunk does not resolve the problem.

This world was created in 1.13.1, but the region file r.1.4.mca was converted from 1.12.2

Can confirm for 1.13.  Attached crash report: 

^{Note, that this is not a chunk with a large coordinate value.  The crash happens when a player loads chunks from position 638, 71, 2254.  Nowhere near out of bounds for these coordinates.}  

Happened on my server when entering the end and the dragon was already dead. The server didn't crash, but the client gets disconnected and is unable to reconnect.

Because the falling item is an entity and entity processing stops when no player is in the dimension this is not a bug. You can work around it by dropping an item through a nether portal within the spawn chunks once every 10 seconds, like you do with an iron farm in spawn. I just added a command to summon an item into a water stream (over ice) that leads the item into the nether portal. This way I don't have to keep my iron farm running all the time. I put this command on the same clock that activates my other command blocks and it solves the problem nicely.

I removed all redstone dust from these observers and removed and replaced the 2 on the right. You see the ones on the right are not powered and the others are stuck in a powered state.

Also, same situation in 1.11.2 without Optifine.

I'm having the same issue. The sugar cane has grown where observer should have seen it and fired. I don't have a screenshot, but the redstone behind the observer was not powered until I broke the glass on top and then it became powered and stayed powered for the entire run of the farm. No repeater loop anywhere as there are no power sources on the redstone dust other than the observers. Seems to have happened after leaving the area and returning after some time.