What is the "it"? Your (1) passport, (2) customs form, (3) some other doc.
In my "X" incident, my passport was not stamped, but (2) was--a gold (or yellow?) line was drawn on that doc--the color I assume has some significance to the CBP agents.
Typically, they stamp the back of the GE paper with the X, to indicate that they have reviewed your case, and you still take that paper to customs.
Cheers,
GenevaFlyer