The issues in an appeal are usually controlled by the appealing party (the appellant), with some modification which arise from the response of the party defending the underlying judgment (the appellee). The appellant makes certain arguments, and the appellee tries to rebut them. Sometimes the appellee also brings up alternative theories in its defense, but these defenses have almost always already been discussed at the underlying trial or hearing. In other words, in broad strokes, nothing should come as a major surprise in the briefs, nor should both parties be surprised by the issues discussed in the Court’s ultimate decision.
But law isn’t as simple as that, and occasionally the Court of Appeals will issue a decision which relies on some issue which neither party ever mentioned at trial or in their briefs. As Donald Rumsfeld famously said:
As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don’t know we don’t know.
The “unknown unknowns” are pretty rare, but they do occur. This can cut both ways. I have had a handful of cases where I won based on an issue which I barely mentioned. And, I have lost based on an issue which neither party even hinted at. Some judges believe it is not their place to fashion an argument for either party, and they avoid interjecting these “new” issues. Other judges are more concerned with “real justice” and will go off the reservation, so to speak. The take away from all of this is that there is no such thing as a guarantee in law. Clients can be frustrated when lawyers refuse to give them “hard odds” of success, but they reality is that the odds are always subject to possibilities, perhaps rare, which are completely unforeseeable.