Wierd Pattern on the Back Wall

Feautures implemented:

• Raycasting from a camera into a scenen through a pixel grid
• Phong lighting for one point light source
• Diffuse lambertian surfaces
• Raytraced shadows
• Cube intersection testing
• Sphere surface point sampling

I don't know what happened to that wall. The diffuse and  ambient

Finally, I found out why. It is because I used EPSILON in the box-ray intersection set when the ray parrelle with one plane. EPSILON is defined as 0.000000001, which seems too small for a float number camparation. I have changed it to 0.001. 
Now, the picutre looks like this: