When only OptiFabric 1.12.10 and OptiFine H5 pre3 are present in mods folder, game crashes with the following error:
The game crashed whilst rendering overlay
Error: java.lang.RuntimeException: Mixin transformation of net.optifine.CustomColors failed
Exit Code: 255

When Fabric API is present, the game reloads perpetually on the logo + loading bar screen.

Crash log:
https://pastebin.com/S7ERUFQM

Seems to be an issue with some sort of new change done to OptiFine's CustomColors functionality. The last compatible OptiFine version with optifabric-1.12.10 is H5 pre2. The latest compatible publicly available version is H4.

Edit: Persists with pre4
Edit: Perpetual reload persists with Fabric API 0.45

Same issue, i gave my details in the above issue before realizing it was a duplicate. Any updates on fixing this?

Same issue, i gave my details in the above issue before realizing it was a duplicate. Any updates on fixing this?

@LegendOfCatBro All you can do is to wait for Chocohead to come back and update their mod.

Indeed for pre4 it keeps reloading forever, on the "final" release H4 works just fine.

Well friends, looks like i stumbled apon this bug too, oddly with 1.12.9 optifabric.
i had updated my optifine because it was needed for a texture pack

Any update? H4 has a problem where you can't see under water with shaders on.

Might be as @RyanCR03 mentioned.
Also, check to see if you can replicate the issue with OptiFine as game loader and/or Forge mod. Then, contact either the shaderpack's creator or open an issue on OptiFine's GitHub. If the issue only occurs on Fabric with that specific shaderpack, create a new issue here on OptiFabric's page.

Any update? H4 has a problem where you can't see under water with shaders on.

i think thats your shaderpack. i use bsl and the water is fine

Same problem persists with preview OptiFine 1.18.1 HD U H5 pre3.
Log: https://paste.ee/p/aByO1

Fixed with OptiFabric-v1.13.0

The game crashed whilst unexpected error
Error: java.lang.NullPointerException: Cannot invoke "net.minecraft.class_1297.method_5705(float)" because "renderViewEntity" is null
Using litematica & Malilib with Optifabric 1.19 and Optifine H9

When Fabric API is present it stops at ~90% on Mojang Studios loading screen, otherwise game starts but when loading into a world game crashes.
Both issues stated persist to this day.