The Rabbi and the Norse King
An official brought the chief rabbi of a town before the Court of the Inquisition and told him, "We will leave the fate of your people to God. I am putting two slips of paper in this box. On one is written 'Guilty.' On the other is written 'Innocent.' Draw."
Now this inquisitor was known to seek the slaughter of all the Jews, and he had written "Guilty" on both pieces of paper.
The rabbi put his hand inside the box, withdrew a slip of paper—and swallowed it.
"What are you doing?" cried the Inquisitor. "How will the court know—"
"That's simple," said the rabbi. "Examine the slip that's in the box. If it reads 'Innocent,' then the paper I swallowed obviously must have read 'Guilty.' But if the paper in the box reads 'Guilty,' then the one I swallowed must have read 'Innocent.'"
It's a clever trick, but I had heard it before:
… it was determined, with the consent of all parties, that lots should be thrown into a box, and the Greeks and Varings should draw which was first to ride, or to row, or to take place in a harbour, or to choose tent ground; and each side should be satisfied with what the drawing of the lots gave them.
Accordingly the lots were made and marked. Harald said to Gyrger, "Let me see what mark thou hast put upon thy lot, that we may not both mark our lots in the same way." He did so. Then Harald marked his lot, and put it into the box along with the other. The man who was to draw out the lots then took up one of the lots between his fingers, held it up in the air, and said, "This lot shall be the first to ride, and to row, and to take place in harbour and on the tent field." Harald seized his hand, snatched the die, and threw it into the sea, and called out, "That was our lot!" Gyrger said, "Why did you not let other people see it?" Harald replies, "Look at the one remaining in the box, -- there you see your own mark upon it."
Haraldsaga was written in the 13th century and describes events in the 11th century—Harald, after serving as the commander of the Varengian guard, returned to Norway, became king, in 1066 invaded England and died at the battle of Stamford bridge. So it is unlikely that either Harald or Snorri Sturluson, the author of the saga, got the idea from Leo Rosten or his unnamed rabbi. The most likely explanation is independent invention; it's a clever idea that could have occurred to two different people.
My preferred explanation, however, is that the inquisitor had the bad luck to pick on a rabbi with a taste for reading the Icelandic sagas.