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X Marks the Spot
Indy defuses several cliches and says "X never, ever marks the spot". Later, the search for the knight's tomb leads them to a library where they realize the tomb is located. The entrance is marked with Roman numerals and the exact entrance is marked with the Roman numeral for ten, which is X.
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Brick Joke
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"X" Marks the SpotExample of:
"X" Marks the Spot