i had a similar experience.
my neighbor is japanese (I live in a building with two departments per floor, he lives in front of me) and i was like all the time asking my friends from the building things about him, (they even give me his newspaper xDDDDDD) and then one day i was with him in the same elevator and he said "good morning" and i was like shocked and said nothing back xD! i was kind of rude... ☝