Person A (who is a smoker): You shouldn't smoke; smoking is bad for you.
Person B: You have no room to talk; you smoke!
In this case, Person B committed the tu quoque fallacy. The fact that Person A is a hypocrite in no way invalidates the truth of the argument. In other words, the argument itself is true, whether the person making the argument abides by it or not.
Here's a popular example of the fallacy I've observed fairly frequently the past few days.
Person A: *Expresses outrage over a perceived injustice.*
Person B: "Were you equally as outraged six years ago when so-and-so from the opposite party did something similar."
Even if the two events are equivalent, and that is rarely the case, the validity of person A's argument stands independent of whether or not they made a similar argument about similar events in the past. People often change stances and viewpoints over time as they learn and grow and experience new things. Chances are you have some different viewpoints on certain issues than you did several years ago. Each event and argument must be considered in its own context. Of course, we ought to strive for consistency, but, and here's the key, the consistency or inconsistency of the speaker neither validates nor invalidates the truth of the argument.
As a pastor, I encounter this fallacy fairly regularly. Just the other day I preached a message about how Christians should respond to refugees and immigrants. Someone who listened to my message and responded by saying that "my rhetoric is empty until" I personally welcomed Syrian refugees into my home. This is an example of the tu quoque fallacy, among others. Whether or not someone has personally hosted refugees is no more relevant to the refugee discussion than whether or not someone has personally adopted children in danger of being aborted is to the pro-life discussion.
Now I want to be very clear about something, I'm not downplaying the seriousness of hypocrisy. Whether it should or not, hypocrisy does discredit our message, especially when it's combined with judgmentalism. I am well aware that Christian hypocrisy is a major stumbling block for non-Christians. Even Gandhi is said to have remarked about the way in which Christian hypocrisy distracted from the teachings of Jesus. Preachers SHOULD practice what they preach; I'm in full agreement with that.
But there's a problem. There has only ever been one preacher who perfectly practiced what he preached, and then he was crucified and resurrected. Every other preacher worth his or her salt will make a hypocrite of themselves, because we are all called to preach a standard that we will consistently fall short of reaching. Bottom line, when it comes to the truth of the message, the integrity of the messenger is nice, but it's not necessary. Truth can, and often does, come from the mouth of a hypocrite. Are we humble enough to receive it?