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Issue description:

Approximately one time in ten, Minecraft crashes during sided_setup (during startup, before you reach the main menu). The crash happens in various different mods, one of which is Mekanism. I have also seen it where Carry On crashes instead.

From Discord: "basically it is a race condition I think where because of parallel mod loading it sometimes happens that two mods try to modify the same map at the same time"

Steps to reproduce:

1. Have Minecraft 1.16.4-forge-35.1.12
2. Install Mekanism (release or git)
3. Run minecraft
4. If it didn't crash, quit and go to 3.

Version (make sure you are on the latest version before reporting):

Forge: 35.1.12
Mekanism: 1.16.4-10.0.17.444, 1.16.4-10.0.18
Other relevant version: For full mods list see attached crash reports

If a (crash)log is relevant for this issue, link it here: (It's almost always relevant)

crash-2020-12-11_00.13.21-fml.txt
crash-2020-12-05_11.27.01-fml.txt

I have not seen this happen since your fix.