A leaking filling refers to a symptom where liquid gets inbetween a filling and the tooth. When a filling is first created it is perfectly fitted to the remainder of the tooth after the cavity was removed. But over the course of a few months or years it is possible for decay to return, especially if you skip dental visits and avoid brushing and flossing. This leads to the tooth eroding while the filling stays the same size and shape. This can create a pocket around the filling. When this happens, a filling can feel loose, or liquid can suddenly drip out from under the filling. If this happens you should see a dentist to have the filling replaced.