Crash dump: https://gist.github.com/Yopu/ba2a71c3b265f571a441

SSP crash, unknown if it affects SMP.
Possible factor: playing with anchor fuel disabled.
Edit: fuel being disabled appears to be the deciding factor.
Edit: it was because it wasn't running otherwise. Doh!

Steps to reproduce:

1. Load world with passive anchor placed.
2. Save and quit to main menu.
3. Reload world without restarting Minecraft.

Found the issue.
Forge appears to not check when you force a chunk if it is already in the forced chunk map.
Solution is checking before we request the unforce operation.

I swear it used to...