If you don't know you're talking to an FBI agent then there's no legal culpability. The poster above you is wrong; it's a felony to knowingly and willfully make false statements to an investigator. So if you don't know who you're talking to you have no obligation to be truthful, likewise it's no crime to make a non-factual statement through simple error rather than to deceive. For example if you tell an FBI agent that your spouse is at work but unknown to you your spouse has left the workplace and gone shopping, that's not your problem.