What's to prevent a rogue Homeland Security agent from falsely declaring someone to be a terrorist?

I don’t think there are any. Not with the Patriot Act. Basically if they want to declare you a terrorist you have no rights. They can put you in a private “prison” and deny you your basic rights. It is a scary thought.

If someone wants to correct me then please do but this is how I understand things.

That’s how I understood it, too. Scary.

There is really no limit to the false positives available in the system, however it also extends to getting flagged as ‘linked to terrorists’ and ‘likely terrorist contacts’ as well. See http://antifascist-calling.blogspot.com/ for more on the shadowlands of this system.

Nothing but the fact they are decent human beings oh dear

Nothing whatsoever !! I had a few relatives that served in the military !! I have been told anyone who significantly outranks you can destroy your life with hardly any evidence or no evidence at all but just their word !
I heard of an Army Captain having an affair with a Sergeant’s wife,to get rid of him the Captain trumped up some BS charges against the Sergeant so he could have his woman,the Sergeant was convicted and sent to federal prison for 2 years on just the Captains word of honor as a commissioned officer !! The truth came out in the Captain’s divorce proceedings 8 years later !!

Nothing at all. And that’s what’s so scary about the PATRIOT Act. If you’re given a National Security letter, you can’t tell anyone about it if they do.

It could be the new McCarthyism!

Absolutely it could.

The lack of checks and balances is really quite frightening.