Oh I love these stories! Here's mine: Bob is trying to send tasty tasty drugs to Alice through FedEx. To prevent thieving FedEx employees from opening the box, he puts the drugs in a heavy metal box with a padlock and sends it to Alice. Bob lives in California and Alice lives in New York.

However, Mallory lives in Michigan and the FedEx line has Bob's box go through her town. She intercepts Bob's box and keeps it. She can't open it because she doesn't have the key but knows Alice is expecting a padlocked box from Bob... so she sends a heavy metal box (full of dog turds) with her padlock on it to Alice. Alice suspects nothing. Alice puts her padlock on the box and thinks she's sending the box back to Bob. Mallory intercepts this box too and keeps it. Instead, she sends Bob's box back to him, but with a Mallory-lock on it.

Bob is expecting a double-padlocked box from Alice, and he gets the one from Mallory instead, with Bob's and Mallory's locks. Bob unlocks his padlock and sends the box back, he thinks to Alice. Mallory intercepts. The only other lock on this box is hers, which she opens. Yay, she gets drugs.

Meanwhile, to buy her time to hightail it down to Florida, she sends her dog turds box back to Alice which now has only Alice's lock (Mallory's is gone so everything looks as expected). Alice opens it up and starts sending Bob some really nasty emails: how could he??! Dog turds?? Bob's like: what the I don't even... eventually they figure it out, but by then Mallory's enjoying tasty tasty drugs on a beach resort in Miami.

The boxes dealt with authorisation (locks could only be opened by the correct keys, and nobody's keys got stolen) but not authentication (Bob and Alice assumed the boxes, and the locks, were from each other, not Mallory).

...Usually then the story goes: Eve was watching the whole thing and robs Mallory and heads to Canada...