Originally Posted by
NewbieRunner
I had to do this once at FRA (duty free liquor bought on board an LH flight coming from an unsafe country

) and they would only accept it if placed in a bag - not just a sealed plastic bag.
Duty free items bought on board an LH flight are something totally different. These are liquids bought on an EU registered aircraft and should be allowed if they are placed in a sealed plastic bag with the invoice shown. However, not every security guy is aware of these rules and not every FA is aware how to pack this properly.
However, liquids bought in a Non EU (incl. Switzerland and Norway) duty free shop or onboard of a third world (sorry, third country) aircraft are simply illegal in hand-luggage when departing from or transfering through the EU.