A Slovak citizen (without his knowledge) had a detonation device planted in his luggage by security personnel as part of airport security test. The device was not found at the airport and the person was arrested later in Dublin after the Irish police were informed about the device.
Source: IDnes news article Jan 5 2010:
Irská policie zatkla Slováka, který nevědomky dopravil do Irska 90 gramů výbušniny, krátce poté ho ale propustila. Výbušninu mu totiž v rámci nepodařeného bezpečnostního experimentu do zavazadla dali slovenští bezpečnostní agenti.
Irish police has arrested a Slovak citizen who unknowingly transported 90 grams of explosives into the Ireland, but released him shortly after. It showed up the explosives were given into the luggage by a Slovak security agents in an experiment which went awry.
The person was released shortly, there was no offence brought against him, but as the question is about arrest, it seems to qualify.
Together with DJClayworth's answer it seems that the risk of being planted a contraband from police / security personel might be comparable or higher to that from a smuggler.