The Librarian worked at the Library of San Barnaba in Venice, Italy in the late 1930s.
One day near closing time, he was working at his desk, stamping books. Several times as he stamped the books, the stamp made a large metallic sound, much more than would be expected from a tiny inkstamp. Puzzled, he set his stamp down.
Unbeknownst to him at the time, Indiana Jones was using a metal cordon post to smash a hole in the marble floor elsewhere in the library, striking at the same time as the librarian was stamping his books.
Behind the scenes
The role of the Librarian, as a non-speaking role, is not credited in Indiana Jones and the Last Crusade.