What sort of shader/texture issue would cause this kind of artifact?

Hi, I'm trying to multi_compile support for specifying a texture in my shader. If I have this code:

     void surf (Input IN, inout SurfaceOutput o) {
         half4 c = _Color;
         #if TEXTURE_ON
         c *= tex2D (_MainTex, IN.uv_MainTex);
         #endif

I get this kind of output:

The weird thing is that the bad texture changes (shimmering differently) every time the camera angle changes, almost as though it was reading uninitialized memory. As soon as I comment out the #if and #endif, the image looks fine:

Any thoughts? Thanks in advance.