Here's another one I use a lot: "I don't know what I don't know."
I've been staying in DVC villas a little more than one week a year since 2008. But, with the exception of one single night renting from another owner, all of those stays were booked as Interval or RCI exchanges. I paid a lot of attention to how the DVC program worked along the way, because that gave me a lot of information about what might be possible as an exchange.
It is fair to say that I probably knew more about the system than many owners, despite never having been an owner myself.
Even so, there have some subtleties in how the various booking, banking, and borrowing rules interact that I didn't quite fully grasp until I started actually using them. I knew what each of the individual rules were, but I hadn't fully worked out how they impact each other, and that led to a sub-optimal decision or two. Nothing that was an "expensive mistake", but I gave up some flexibility earlier than I needed to.