I once had the "honour" of being the bearer of a sealed briefcase to bring back to UK which contained the details of the nefarious activities of certain people who resided in an island close to (but just west of) the UK.
HM Customs : "Please open that briefcase Sir"
Me: NO (producing document which asserted that the document was not liable to Customs search).
Customs: I will have to detain you.
Special Branch: Oh no you ******* won't.