Apollo 15 postage stamp incident

The Apollo 15 postal covers incident, a 1972 NASA scandal, involved the astronauts of Apollo 15, who carried about 400 unauthorized postal covers into space and to the Moon’s surface on the Lunar Module Falcon. Although it was not publicly known until September 1972, 15 of the men who entered space as Apollo program astronauts before Apollo 15 had agreed with a West German named Horst Eiermann to autograph 500 philatelic items (postcards and blocks of stamps) in exchange for $2,500. When Sieger learned that Eiermann knew many astronauts, he proposed that an Apollo crew be persuaded to take covers to the Moon.

Source: en.wikipedia.org