The blast furnace preheater has some interesting shading on it when using OptiFine. This doesn't occur when not using OptiFine. Not sure if this is a bug with OptiFine or IE.

When using OptiFine, the blast furnace preheater looks like this:

Without OptiFine, the preheater looks like this:

Versions & Modlist

MC: 1.12.2
Forge: 14.23.5.2768
IE: 0.12-88
OptiFine: 1.12.2_HD_U_E3
CTM: 0.3.3.22

Optifine is not supported. Says so quite clearly in the contibution guidelines.