The best laid plans can still go awry. Things pop up. Things can go wrong. As a result, expect and prepare for the unexpected.
One way to prepare for the unexpected is to give yourself a buffer. The buffer can relate to timing, quality, or some other factor. But, always give yourself a buffer in order to deal with the unexpected in life. Under-promise and then, if things fall into place, you get the pleasure of the “wow” factor and can over-deliver. If things fall apart, then you have the opportunity to deliver what you promised (and the buffer can eliminate some stress in your life in the process).
Whether with respect to your boss or a client, give yourself a buffer just in case you find yourself having one of “those days.” Under-promise and then do your best to over-deliver!