Fate Core says this about contests:
Whenever two or more characters have mutually exclusive goals, but they aren’t trying to harm each other directly, they’re in a contest.And this about conflicts:
As long as the characters involved have both the intent and the ability to harm one another, then you’re in a conflict scene.
Taking the example of a possession, my assumption has been that this needs to be a conflict. It's a mental assault and when the target is Taken Out, they are possessed. This dates back to when I first started futzing around with Spirit of the Century and I've carried it through to Strands of Fate and started on that foot with Fate Core. Now this method is perfectly valid - but it still means that logically the character needs to be Taken Out, and that means they'll have a lot of consequences. For some games, having a totally beat up possessed character is likely perfectly fitting. For others, it goes against how something like possession is assumed to work - there's no real damage to the character, they're just not in control of their actions. This has been a niggling point for me from the very beginning.
But then I realized the key phrase in the description of contests: "they aren't trying to harm each other directly". The character doing the possessing isn't a trying to hurt the possessed. Just get control of them. Which sounds like placing an aspect on them, which is perfectly within the definition of a contest. Once I started to think this way, a number of things clicked into place and I was able to rewrite a few stunts.
Luckily, I've seen a couple other things that people have written that include contests used in this manner so I feel like it's the right solution to that sort of problem. It also shows that Fate Core continues to deliver revelations about it's flexibility and depth, even eight months into working with it.