Welded parts should always be unanchored, this is obvious, because the idea of welds is to make two moving parts stick to each other.
But I think a beginner would have a hard time if he doesn’t know this, he can have two welded parts where one of them is anchored and the welds would not work, and he can be stuck for hours. The wiki page doesn’t cover that piece of information, in my opinion I think this should be added.