Forge version:

14.23.5.2855

HAC Mod version:

1.12.2-3.6.8

HAC Lib version:

1.12.2-3.6.6

Crash report:

https://paste.dimdev.org/qojaqosisa.mccrash

How to reproduce:

1. Run command /effect @p dcs_climate:dcs.potion.absorption_exp 255 255, or equip a Black Golden Ring.
2. Summon enough mobs. You may use a command block with command
   summon enderman ~ ~1 ~ {NoAI:1}
   to get plenty endermans for testing, due to their Exp drop is more than other mobs.
3. Switch to Survival Mode, then kill them. To reproduce this issue, you may need to kill mobs as many as possible in one attack. In my testing, I used the sweeping attack of a SlashBlade Mod's weapon.
4. Game crashes.
5. Install CodeChickenLib mod.
6. Retry Step 1 to Step 3.
7. Game will not crash.

Thanks for your help in advance!

Thanks for the report.
This bug has been fixed in 2b64899.