"And oftentimes excusing of a fault doth make the fault the worse by the excuse"

This quote by William Shakespeare is suggesting that when we attempt to excuse our mistakes, we can commonly make the mistake even worse. This is due to the fact that when we attempt to explain away our blunders, we can wind up making justifications that make the fault seem even more serious than it really is. For instance, if someone makes a mistake and then tries to discuss it away by stating that they were exhausted or distracted, this can make the error appear even more severe than if they had merely accepted responsibility for it. In other words, when we attempt to excuse our mistakes, we can typically make them even worse.