There is an achievement in The Cave called "Shoplifting". The achievement text says:
Saw a postcard stolen from the Gift Shop.
I assumed it meant the postcard you could pick up from the Postcard Display next to the Gift Shop Clerk.
However, I took the postcard from said display, carried it around with me for quite a while, and never got the achievement. This leads me to believe I either a) did something incorrectly, or 2) mistook this postcard for one later in the game.
How do I get the Shoplifting achievement?
Best Answer
Just at the beginning of your tour into the cave, take a postcard from the gift shop and go with it into the cave (after you gave the guy these three objects from the very first puzzle). The achievement pops right after you get through the door.
Do note there is another achievement following this one called Remorse
Source: Shoplifting Achievement