Grainy image in my simple ray tracer
It's likely due to shadow rays. When the light source is exactly on the plane of the wall, a shadow ray traced to it may or may not be found to intersect the wall before it reaches the light. The result will depend on arithmetic roundoff error, and may appear as random noise, or stripes or some other artifact.

To fix it, you can either keep the light offset slightly from the plane, or set up the wall object not to cast shadows (assuming it doesn't need to in the scene).