Facebook has access to an unprecedented amount of your information, and it often finds itself under fire for what it does with that data. However, Facebook isn’t the only party interested in gathering details about you online. Increasingly, law enforcement agencies are using the platform to keep tabs on users, search for evidence, and even potentially use people’s own words against them. That’s what happened recently when New Hampshire man Robert Frese was arrested for calling a police officer a “dirty cop” on Facebook.

According to a report in The New York Times, Frese was arrested in Exeter, New Hampshire, for violating the state’s criminal libel laws. These are tough cases to prove, because the law requires that prosecutors prove the defendant knew he or she said something false. However, experts say that lack of clarity is part of the problem.

“The fundamental defect of criminal libel statutes is that they’re unconstitutionally vague,” Brian Hauss, a lawyer for the ACLU, told the Times. “The practical result of that is that police departments, like the police department in Exeter, get to choose when they want to go after speech that is arguably defamatory.”

Beyond the serious legal ramifications, cases like this prove that it pays to be careful of what you say on Facebook. You never know who could be watching — besides Facebook itself, of course.