When trying to drag tabs from a window in Opera to open them in a new window, I find it rarely works. I click and drag, and the tab stays there. What I have found to happen happen, is when I am trying to re-order tabs, then the tab I am trying to re-arrange will more often than not pop out of the window. When the tab is popped out of the window, there is no way to put it back.
So I guess from now on, if I want to drag a tab from a window, I'll just try to re-arrange it instead, but overall this seems to be a very poorly functioning system. Tabs should pop out when dragged down. Tabs dragged left and right should stay in the window. And let us put tabs back in the window, if they pop out for no good reason.