I forgot about this one -- Apollo 11:

109:49:40 Aldrin: Hey, Neil, didn't I say we might see some purple rocks?

109:49:42 Armstrong: Find a purple rock?

Aldrin - "The statement about 'purple' rocks was really a stretch of being kind of facetious (that is, he was joking). But it didn't come off. Since it didn't come off well, I let it go at that. What I was kind of thinking of was 'what's the most absurd color you could think of for a rock?' 'Purple.' But it just didn't come out right."]


http://www.hq.nasa.gov/alsj/a11/a11.step.html