In Indiana Jones and the Last Crusade, Indiana Jones is doing a lecture in his classroom, explaining to his students that X never marks the spot:
Archeology is the search for fact, not truth. If it's truth you're interested in, Dr. Tyree's philosophy class is right down the hall. Forget any ideas you got about lost cities and exotic travel and digging up the world. We do not follow maps to buried treasure, and X never, ever marks the spot
Later in the movie, he is looking for clues in a library with Dr. Schneider and comes upon the Roman Numeral X. After seeing this, he states:
X marks the spot!
He then proceeds to excitedly start "digging" where the X is marking the spot
Why did Indiana Jones immediately start digging where the X marked the spot when he specifically told his students that X never marks the spot?