1.61. A man is riding a subway. He meets a one-armed man, who pulls out a gun and shoots him. (SJ; from How Come?)
1.61 answer: Several men were shipwrecked together. They agreed to survive by eating each other a piece at a time. Each of them in turn gave up an arm, but before they got to the last man, they were rescued. They all demanded that the last man live up to his end of the deal. Instead, he killed a bum and sent the bum's arm to the others in a box to "prove" that he had fulfilled the bargain. Later, one of them sees him on the subway, holding onto an overhead ring with the arm he supposedly cut off; the other realizes that the last man cheated, and kills him.
1.61a. Variant wording: A man sends a package to someone in Europe and gets a note back saying "Thank you. I received it."
1.61a answer: This is just a simpler version; the shipwreck situation is the same, and the man actually did send his own arm.
1.61b. Variant wording: Two men throw a box off of a cliff.
1.61b answer: Exactly the same situation as in #1.61a (one slight variation has a hand in the box instead of a whole arm), with the two men being two of the fellow passengers who had already lost their arms.
1.61c. Variant wording: A man in a Sherlock Holmes-style cape walks into a room, places a box on the table and leaves.
1.61c answer: In this one he's wearing the cape either to disguise the fact that he hasn't really cut off his arm/hand as required, or else simply in order to hide his now-missing limb. (from Joe Kincaid)