Update your twitter in class and you might get a glare from your professor. Tweet at a G20 Summit protest and you could get arrested.

That’s exactly what happened to Elliot Madison—a 41-year-old, self-described New York Anarchist—who allegedly used Twitter to advise protesters at the Pittsburgh economic summit where police would be stationed so they could evade the officers.

Madison now faces charges of hindering prosecution, the Huffington Post reported Saturday.

FBI agents obtained a search warrant of Madison’s home Thursday. According to court papers filed by his attorney, the agents confiscated computers and leftist political writings from Madison’s home in Queens, New York.