LPT: You can prevent unpleasant situations with unexpected visitors by simply not answering your door. You're under no obligation to open it unless it is a cop holding a physical warrant. If someone really wants to get a hold of you, they can use the mail or other legal channels.

If a cop says they have a warrant, how do you know? Have them press it against the window so you can read it. No windows because you're in an apartment building? Have them slide it under the door.