If you, say, fall at terminal velocity through not-yet-generated chunks for long enough, the server thread will just die; afterwards, any attempt to "save and quit" freezes the client. Tested in single-player, both opened to LAN and not; haven't had a chance to test a dedicated server yet.

[16:27:44] [main/INFO]: [CHAT] Teleported <PROFILE NAME> to 10000.5, 2500.0, 10000.5
[16:27:54] [Server thread/ERROR] [cubicchunks]: Attempting to unload already unloaded cube at CubePos(619, 4, 622)
[16:27:54] [Server thread/ERROR] [cubicchunks]: Attempting to unload already unloaded cube at CubePos(620, 4, 621)
[16:27:54] [Server thread/ERROR] [cubicchunks]: Attempting to unload already unloaded cube at CubePos(624, 4, 631)

this happened to me in singleplayer, annoying af

Can confirm that this also happens in dedicated servers.

I tried and I still can't reproduce the issue.

My setup: new CustomCubic world, forge build 2600, cubic chunks 0.0.824.0.

Flied 10000 blocks away from spawn on spectator mod on max speed, in the meantime switching render distance values, sometimes going back and forth, changing directions and going in circles.

At that speed I was surprised cubic chunks works as well as it does anyway, but no issue found.

Are you using other mods? Which cubic chunks version? Which forge version?

I was able to reproduce the issue with JourneyMap installed. JourneyMap causes chunks to be generated outside of server thread, which completely breaks XYZMap class, causing the iterator to hang while looking for next element. As it is, this is not fixable and not considered an issue with cubicchunks, it's just journeymap being incompatible.