In Nigeria they are currently doing the 'Yellow Fever' trick just now.
The 'Port Health' inspectors in lab coats ask you for your 'yellow fever' medical certificate even if you have come from somewhere that has never had Yellow Fever.
Funny that they only ask certain groups of persons? If you don't have the certificate then it's $50 for the piece of paper (no inoculation), although you can get it down to $20 with a good ol' debate.
I even had copies of WHO advice, Nigerian Government & British Government advice saying that I don't need Yellow Fever as my country isn't affected, etc. The "Inspectors/Dr's" aren't interested.
I was recently asked on the way OUT of Nigeria for my yellow fever certificate.
Welcome to Nigeria.