1. Not changing your password regularly. You've probably heard it a million times, but it's THAT important. If you've been using the same password for five years, change it. Especially if you use the same one across multiple websites.
2. Posting a photo of your house key. Some people do it to celebrate after they get a new place. But hackers can use that photo to make a copy of your key and break in. And if the photo is geo-tagged, they can find out exactly where you live.
3. Your teenager posting a photo of their new driver's license. As dumb as it sounds, it happens all the time. And hackers can use it to steal their identity, or go after YOU since you're the one with the money.
4. Not looking at what's in the background of photos before you post them. Like taking a selfie at your desk, and not realizing your passwords are taped up behind you. And also be careful about things like paychecks, and what's on your computer screen.