Shader looks different in game than in scene view

I have a shader I made about a year ago, that worked fine then, but acts weird now. It's a front culling shader, and in the scene view it looks correct, but in the game view the frontfacing triangles cull the backfacing triangles.

Any idea what could cause this?

Edit:

the shader was made in unity 3.5 btw, and I'm using 4 now

I noticed it works when I swith to Deferred rendering instead of forward, or when I disable every post effect.

So it appears the problem only appears in combination with forward rendering and post effects. weird

here's the (simplified) shader:

 Shader "Custom/Test" 
 { 
     Properties 
     {
         _MainTex ("bidon bedrukking", 2D) = "bidon bedrukking" {}
     }
 
     Subshader 
     { 
         Tags 
         { 
             "RenderType" = "Opaque" 
         }
 
         LOD 300
         Cull Front
 
         CGPROGRAM
         #pragma surface surf Lambert
         #pragma target 3.0
 
         sampler2D _MainTex;
 
         struct Input 
         {
             float2 uv_MainTex;
         };
 
         void surf(Input IN, inout SurfaceOutput o) 
         {
             float2 uv = IN.uv_MainTex;    
             fixed4 tex = tex2D(_MainTex, uv);
             o.Albedo = tex;
             o.Alpha = 1;
         }
 
         ENDCG
     }
 
     FallBack "Diffuse"
 }