Description of the issue:

Occasionally when placing slabs, the game says "internal server shutting down" and then crashes.

Crashlog:

https://pastebin.com/y3NWkeM5

Versions & Modlist

IE version 0.12-82
Forge 1.12.2-14.23.4.2705

i could swear there is some optifine in the trace but i can't spot it in the list... PlayerOF was it?

Lines 128, 227, and 228. Sometimes its not in the mod list but still installed, I just do Ctrl+F

Yeah, I don't think Optifine is hacky enough to interfere with block placement. It merely does visual and optimization changes.

And yet it is part of the crash and the contribution guidelines ask you to repeat the issue without it

I'll try without Optifine, but since I'm not going to be placing slabs much, it won't be too big of a deal. Besides, I'll be updating soon anyways.