...it's easy to get your fingers burned. I was looking at an issue with personalisation recently, which reminded me that those little details in the docs, which change between versions can often be the most important ones.
I'm working on my first proper Sitecore 9 project at the moment, and got bitten by an annoying bit of confusion while doing some configuration work. If you're tweaking how xConnect works take note, and hopefully you can avoid making the same mistake I did...