Seeing all the photos you’ve ever liked or commented on... ever
Thanks to good ol' Graph Search, you can see almost all the photos someone has liked or commented on. Simply search for "photos X person has liked" or "photos X person has commented on," and it'll cull a gallery. This is terrifying on so many levels, especially if you have one of those less-than-trusting significant others, but also because it makes it easy for a creeper to identify who you're closest to.
Getting notifications for everything you do on FB
The "Close Friends" feature allows your current friends to keep uncomfortably close tabs on you. Every time a Close Friend does anything on Facebook -- likes or comments on a photo, updates their status, checks in somewhere, etc. -- you'll get a notification. But since Facebook doesn't tell people when they've been added as a Close Friend, there's no way of knowing who's getting an up-to-the-second broadcast of your online behavior.