"email from FBI", and the Nigerian FBI office at that ... Reminded - a professor of a Moscow University couple months ago received a call from Russian Central Bank advising him that his account in some bank is being actively targeted by scammers/hackers, and that he needs to temporarily transfer the money to the special holding account the Central Bank rep provided, so the professor did. Some time later the scammers started to target the professor's condo - the police agent called him informing about it and asking for help to catch the scammers - when the scammers come with the prepared documents for the condo sale, professor would need to play the part as if he doesn't know what it is a scam and to sign the documents, receive the money and after that to give the money as evidence to the special agents in the car near the condo building. And professor did as he was told. So far - no money, no condo, no bank account with the significant sum of money...

Or as our corporate anti-phishing/etc. training - which was forced again upon us last month - instructs "Got a call from John from company A ? Hang up and call the public phone number of the company A and ask for the John."