Or, say you're out on the town one day and your drunken best friend goes to urinate in an alley, because maybe he has a phobia of public restrooms. A policeman asks you about it later, at which point you swear you have never met the pissing man in your life.
The cops then go into your Facebook profile, find the pissing man on your friends list, and charge you with obstruction of justice.
Others have failed to get away with youthful shenanigans when pictures were posted of them charging onto a field after a football game. Then you have the students who have faced suspension or expulsion for making derogatory comments online about campus security online. It's become so common that in 2006, students at George Washington University decided to turn the tables. They deliberately bragged about an upcoming party on Facebook, waited for police to storm the place, and then revealed that they were all actually eating cake out of beer cups.