Tu Quoque is when you dismiss someone's criticism by saying that the criticizer has the same flaw. Everyone thinks it's a fallacy, but it's often not. The examples that people give for it are usually ones where the criticizer acknowledges that they have the same flaw (eg. someone saying "I shouldn't be doing this but I'm selfish, don't be like me"). In these cases it's a fallacy. But if someone who does X and stands by it says X is wrong, they are indeed contradicting themselves and it's fair to dismiss their arguments. It makes no sense to say that my opponents have to deal with my arguments validly while I don't. I don't get a free pass just because I shot first.
Another way to look at it is this: if I refute an opponent's position, but my opponent shows that my own position is also incompatible with the principle I used to do it, then both of us have been refuted. I do not win the argument.