To Repro:
1. Open the attached project i OS X and scene.
2. Notice the cube with the green leafs doesn't have fog
3. Open the "GlobalUVMappedDiffuse shader
4. Look at the commented lines for a workaround and cause of the bug:

//o.uv = screenSpaceUVs(mul (UNITY_MATRIX_MVP, i.vertex)); // NO BUG
o.pos = mul (UNITY_MATRIX_MVP, i.vertex);
//o.uv = screenSpaceUVs(mul (UNITY_MATRIX_MVP, i.vertex)); // BUG, reuses register
o.uv = screenSpaceUVs(o.pos); // BUG

The problem seems to happen when the o.pos value is used later in code.