Under-tightening or over-tightening the dome on the camera can cause IR ghosting and seal issues which can lead to water or debris penetrating the dome. IR ghosting happens when the light from the IR lights is reflected off the dome and back into the lens of the camera. This will often result in the night time image having small dots of light or blurry spots or areas in the image. Since this has to do with the IR lights, the issue is not always apparent in the daytime image. If there are other light sources present in the area, it is also possible that light is filtering in through a gap and reflecting off the dome into the lens of the camera.
Example of Light Filtering
To allow for even distribution of pressure, tighten the screws in rotation: tighten one screw with a few quarter-turns and then go to the next screw rather than tightening one screw at a time and screwing it in completely. This ensures an even tightening of all screws and prevents light from filtering in through a gap.