Memory leak causes minecraft to consume all available memory.
This can cause computer to swap excessively / hang.
Seems to happens when Custom NPC GUI left open or player is afk.

Issue is with Custom NPC mod CustomNPCs_1.12.2(30Jan19) & iChunUtil-1.12.2-7.2.1
Issue is fixed in next release. ETA unknown.

Icek00l
I don't know if this is something that has been reported before, or is even a common issue, but with Custom NPCs, whenever I spawn even one NPC, my minecraft client begins to just keep using more and more memory until my computer crashes and runs out of RAM. Minecraft does not generate a crash log, in this case, and so I have no idea in the slightest what might be causing this to happen.
This is for the latest version of Custom NPCs for 1.12
Sakuya Izayoi
do you also have iChunUtils?
if yes its a known issue that will be fixed next release
Icek00l
okay thank you
Also yeah, that appears to be the case :P

Work around: Restart client will clear memory. Limit AFK when interacting with NPCs.

does not seem to affect players -- only a problem when doing world editing
so I no longer care about this issue

Work Around 2: Disable mods iChunUtil and Portal Gun

This will remove content from the game but it is not crucial to gameplay