Taking wads of fake cash across the border. Legally!
Reminds me of an incident early in my career - late 70's or early 80's - when a colleague had to explain to French customs why he was carrying a suitcase full of fake banknotes into Orly Airport.
We were working for a company making banknote sorting machines (DeLaRue), and the machines were marketed on their ability to detect fakes.
The Bank of England keeps stocks of banknotes in most of the world's currencies, both genuine and forged, for reference purposes, and the company had an arrangement with them to borrow these for calibration purposes.
We had just sold a machine to a French bank and the engineer was flying over to commission it. As per our usual arrangements, he called in to the BoE to collect the mixture of real and fake banknotes kept for this purpose. He then phoned his contact in Paris to arrange a lift from the airport.
Unfortunately, the call was overheard and reported to the French police. As a result, he was arrested as soon as he landed. The customs officer asked him what was in the suitcase and he replies, quite truthfully "Forged banknotes".
He spent a couple of days in the cells before it was all sorted out and they managed to convince the police that, in this case, it was actually quite legal to be carrying forged money around!