When you take long exposure shots at night with a UV filter (or other flat filter) on the lens, strong light sources such as the moon may reflect off some flat glass inside your lens or even off the glass on your sensor inside your camera, bounce back to the backside of your UV filter and reflect back in to the sensor and leave a ghost image in your shot.
This is not one of those faint theories that might affect something no one really sees, this really destroys your shot. Just look at the image below.
After removing the UV filter the shot came out like this. Still not a great shot, but at least that is not the equipment's fault.