Lateral displacement source of error?

hokay, so, we recently did a refraction lab in physics. We put a semi circular dish on polar co-ordinate paper, with the center of the dish in the middle of co-ordanite thing. We then shone a ray of light from a ray box into the dish at the center of the flat side and measured the angle it came out at. One question asks if the lateral displacement from the plastic dish is significant source of error? I think it's very small, because it refracts when it hits the plastic,right? but wouldn't that undo when thaat undo when the light goes from the water to the plastic back to air?