What's going on with this render?

What’s going on with the stained glass texture on the windows seen on the right here? It’s an image texture plugged into the color of the principled shader with the transmission set to max so that the light from outside can shine through.

You’re probably using single planes as windows… ‘Transmission’ controls the refraction based on the IOR, which depends directly on the surface normals. Because of this, you get a ‘total internal reflection’ (google it).

Solution 1: use transparency instead of transmission

Solution 2: invert the IOR value (1/IOR) if Geometry.backfacing is true