Everyone dieter knows the single cookie syndrome. You are good for three days, and then you eat a cookie. Since you have broken the diet, you may as well go ahead and have the cheesecake and tiramisu.
All or nothingism—this way of thinking touches many things in our life. If you aren't perfect, all is wasted. This is behind a lot of misuse of the word hypocrite. When someone says that they believe in being nice and then does something unkind for example, we call them hypocrites. When candidates for office live in ways that fall short of the ideals they espouse, we call them hypocrites. A healthier way of thinking about human nature is not that we are hypocritical, but that we are imperfect. A misstep does not ruin a character any more than a cookie torpedoes a diet.
You can say a nasty thing and still be a nice person who is not perfect. As parents we often tell our children to behave in ways we deem appropriate only to have them point out that we do not always fulfill the advice we proffer. A wise parent reminds a child that we do try but our ideals should be greater than our capacity to fulfill them—who wants to aim so low as to live perfectly in line with their ideals? Real hypocrisy is saying something or holding up a standard for others that you do not believe or even try to meet. Imperfection is promoting ideals you try to meet but cannot; that is also called being a person.
Of course there is a stuffy self-righteousness that invites the charge of hypocrisy. When you pretend you are perfect, others will delight in proving you are not. When you are harsh about the missteps of others you set yourself up as a target. When you repeatedly violate the beliefs you espouse, "imperfect" is no defense. But an honest commitment to standards that we cannot quite reach is not hypocritical. In every religious tradition there is an assumption that people will falter, and fail. That does not make us evil—it makes us human.
The dieter would be better served by understanding that life is not a zero sum game. As with eating, so with life: it is an incremental journey, with steps forward and back, and the accumulation over time is what counts. Jewish philosopher Franz Rosenzweig taught that when someone asks if you perform a certain mitzvah (Divine commandment) and you don't, do not say "no" but rather, "not yet." Of course he intended that you mean it. "Not yet" is not a dodge to get off the hook, but an indication of a commitment to growth. As a Rabbi I preach standards that I often do not meet. But I believe in them, and try to do better. We should be self-forgiving strivers. And one cookie couldn't hurt.