Although it works surprisingly well in real life, covering mistakes and other follies will only make your task of completing tetrises that much more difficult. On your left is a slightly exaggerated example of what may happen when you cover one gap too many.
The occasional gap coverage, however, is often a necessity for ultimate survival. It is usually best to simply uncover it by scoring singles or doubles immediately afterward. One should not to try to go "all out" and try to score a tetris to uncover the gap; usually that only worsens things. And try never, ever to stack more blocks on top of the gap.