What are the common mistakes PhD candidates make in their final defense session? For a PhD candidate that has done a fine job in their PhD, what are the common mistakes to avoid in a defense session?
How the dissertation defense works varies enormously between fields, countries, universities, and departments.  Any guidance has to take into account what the expectations are.

For example, in the defenses I'm familiar with, there are no common mistakes at all.  In fact, I'm not sure I've ever seen anyone do anything worth calling a mistake.  Here, the defense is basically a formality: you don't reach that point unless your advisor and readers are happy with your dissertation and confident you will easily pass, and in practice nobody fails unless something goes dreadfully wrong.  That can happen (I know of one case in which a serious mistake was uncovered at the last minute, which was certainly embarrassing to both the candidate and the committee), but I've never actually been present when someone failed.  So if someone from my department asked me about common mistakes they should avoid, I'd tell them not to worry about it: making it to the defense is the hard part, and then passing it is really straightforward.  It's more of a celebratory ritual than an actual test.

But of course this depends on how the defense is handled in your department.