When you say something by accident - something which you did not mean to say - you have made a slip of the tongue. Example: "I didn't mean to say that! It was just a slip of the tongue." When you are walking and one of your feet slip, you lose your footing and sometimes you fall down; when you make a slip of the tongue, you misstep with your words (the tongue) and say something that you did not plan to say. Example: "Be careful talking to the police tomorrow; one slip of the tongue could get us into big trouble." Sometimes a slip of the tongue causes you a problem because you have said the wrong thing at the wrong time.